build: unify the gate so root make check covers the backend (closes #16)

Root `make check` only ever ran the frontend, so the "main is always
green" policy was satisfied vacuously: the Go backend could be entirely
broken and the root gate stayed green.

- The backend moves onto scripts-to-rule-them-all. Its test, lint, fmt,
  fmt-check, build, run and clean implementations now live in
  `backend/script/`, and `backend/Makefile` is thin shims. The backend
  is its own project (own module, README, LICENSE, linter config,
  Dockerfile stage), and `Dockerfile.backend` only copies `backend/`
  into its builder, so its scripts have to live under `backend/`.
- The root `script/test`, `script/lint`, `script/fmt` and
  `script/fmt-check` now run the frontend step and then the matching
  `backend/script/*` step, so `script/check` — and therefore the
  pre-commit hook — gates both halves. The frontend-only steps moved
  into `script/frontend-*` so nothing is duplicated.
- `script/bootstrap` now provisions the backend's toolchain as well,
  because widening the gate without widening bootstrap left the
  documented fresh-clone path (`make setup`) installing a pre-commit
  hook that rejected every commit with `golangci-lint: not found`.
  golangci-lint is installed at exactly `2.7.2`, the version
  `Dockerfile.backend` pins, so local findings match CI. Go is reused
  only when the installed version falls inside a window — at least
  `backend/go.mod`'s floor, and no newer in major.minor than the Go the
  pinned linter was built with — otherwise `go1.25.7` is installed. The
  upper bound is load-bearing: golangci-lint links `go/types` from its
  own build toolchain, so the pinned `2.7.2` (built with `go1.25.4`)
  panics with "file requires newer Go version go1.26" against a host Go
  1.26, which would leave `make setup` exiting 0 and every commit
  rejected. Both tools come from a specific release archive whose sha256
  is hardcoded here and verified before anything is unpacked — never an
  install script piped to a shell — and both are symlinked onto `PATH`,
  since nvm-style activation does not reach `make` or the git hook.
- `script/bootstrap` links only into `~/.local/bin` and never into a
  system-wide prefix. `/usr/local/bin` is shared with other users and
  with a package manager — on an Intel Mac it is the Homebrew prefix —
  and pointing an entry there at one user's `$HOME` breaks it for
  everyone else. It also refuses, non-zero, to replace anything it did
  not create: only a symlink already pointing into its own toolchain
  directory is overwritten, so a pre-existing binary is reported rather
  than deleted. `corepack enable` is given `--install-directory` so its
  four shims (`yarn`, `yarnpkg`, `pnpm`, `pnpx`) land inside that same
  toolchain directory instead of beside the corepack binary, and only
  `yarn` is linked onto `PATH`.
- `script/bootstrap` exits non-zero when it cannot guarantee the pinned
  toolchain is the one the gate will run. Reporting success while
  knowing a different linter or a newer Go precedes `~/.local/bin` is
  the same defect this commit exists to remove, so the final step
  re-resolves `go`, `gofmt`, `golangci-lint`, `node` and `yarn` against
  the caller's own `PATH` and fails with what it found and how to fix
  it.
- `script/frontend-check` is the frontend half of the gate, exposed as
  the `frontend-check` target, for the frontend Dockerfile: its build
  stage is a node image with no Go toolchain. The backend half is gated
  by `Dockerfile.backend`, and `script/cibuild` builds both images, so
  the two Dockerfiles together still gate the whole repo. The
  `backend-check` target is the mirror of it. Both targets are named
  after the script they shim, like every other target.
- `script/cibuild` builds both images through one `build_image` helper,
  and the Gitea workflow's only build step is `script/cibuild`; the raw
  `docker build -f Dockerfile.backend .` is gone from the workflow.
  `script/docker` likewise builds and tags both images.
- `backend/Makefile`'s `hooks` target is removed. It wrote the same
  `.git/hooks/pre-commit` as `script/install-precommit`, so the two
  clobbered each other and the developer silently ended up gating on
  only one half of the repo. `script/install-precommit` is now the only
  installer, and the hook it writes runs the repo-wide `script/check`.
- `backend/Makefile`'s `docker` target is removed too: the backend image
  builds from the repo root with a root-level Dockerfile, so it belongs
  to the root `script/docker` and `script/cibuild` rather than to a
  backend script that would have to reach outside `backend/`.
- `backend/script/lint` verifies that `.golangci.yml` still matches its
  pinned sha256 before running the linter. Offline hash comparison, no
  network. The pin is marked provisional in the file: it is the config
  currently on `main`, and the comment names PR #31 and the canonical
  hash that must replace it when #31 lands.
- Every script locates the repo root with the mandated
  `$(cd "$(dirname "$0")/.." && pwd -P)` idiom, `cd`s there, and calls
  siblings as `"$ROOT/script/<name>"`; the `SCRIPT_DIR` variant is gone.

READMEs at the root and in `backend/` document every script, the
backend's Getting Started separates commands run from `backend/` from
those run at the repo root, and `TODO.md` records the change.

TODO.md

@@ -28,7 +28,12 @@ files, so merging it also closes most compliance gaps.
   pre-commit hook installer in `backend/Makefile` was removed, `script/cibuild`
   now builds both images as the workflow's only build step, and
   `script/bootstrap` provisions the backend toolchain (pinned, hash-verified Go
-  and golangci-lint) so a fresh clone can pass the widened gate
+  and golangci-lint) so a fresh clone can pass the widened gate. Bootstrap
+  matches the Go pin rather than treating it as a floor, because the pinned
+  golangci-lint cannot analyse packages built by a newer Go; it links only into
+  `~/.local/bin`, never a system-wide prefix, and refuses to replace anything it
+  did not create; and it exits non-zero rather than reporting success when the
+  tools on the caller's `PATH` are not the pinned ones
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints, Makefile
   shims, README Entrypoints section
 - 2026-02-27: backend with buffered zstd-compressed report storage; CI workflow

script/bootstrap

@@ -16,13 +16,35 @@
 # pre-commit hook that script/setup installs.
 #
 # Anything installed outside the system package manager is symlinked
-# into a directory that is on PATH, so a later `make check` in a plain
+# into ~/.local/bin, so a later `make check` in a plain shell finds it.
-# shell finds it. nvm only puts node on PATH for shells that source
+# nvm only puts node on PATH for shells that source nvm.sh, which
-# nvm.sh, which neither make nor the git hook does.
+# neither make nor the git hook does.
+#
+# Three rules govern what this script is allowed to touch:
+#
+#   1. It never writes outside $HOME. A per-repo bootstrap has no
+#      business writing to /usr/local/bin, a Homebrew prefix, or any
+#      other system-wide location shared with other users and with a
+#      package manager.
+#   2. It never replaces something it did not create. Only a symlink
+#      that already points into one of its own managed directories is
+#      overwritten; anything else is left alone and bootstrap exits
+#      non-zero telling you what to remove.
+#   3. It never reports success while the tools a later `make check`
+#      would pick up are not the ones it provisioned. If it cannot
+#      guarantee the pinned toolchain wins on your PATH, it exits
+#      non-zero rather than leaving you a green bootstrap and a broken
+#      gate.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
+# The PATH as the caller had it, captured before ensure_bin_dir amends
+# it. verify_toolchain checks against this rather than against the PATH
+# this script builds for itself, so what it reports is what a later
+# `make check` in the user's own shell will actually resolve.
+ORIG_PATH="$PATH"
+
 # Pinned versions, 2026-07-07
 NODE_VERSION="22.17.0"
 NVM_VERSION="0.40.3"
@@ -32,10 +54,27 @@ YARN_VERSION="1.22.22"
 
 # Go 1.25.7 (2026-08-09). This is the toolchain inside the pinned
 # golang:1.25-alpine builder of Dockerfile.backend, so a local build
-# uses the same compiler CI does. GO_MIN_VERSION is the floor from
+# uses the same compiler CI does.
-# backend/go.mod; an already-installed go at or above it is used as is.
+#
+# The Go pin is a compatibility constraint to match, not a floor to
+# clear. golangci-lint links go/types from the Go release it was built
+# with, and go/types refuses to load packages compiled by a newer Go:
+# with the pinned linter (built with go1.25.4) and a host Go 1.26,
+# `make check` dies with
+#
+#     panic: file requires newer Go version go1.26
+#            (application built with go1.25)
+#
+# So an already-installed go is reused only inside a window:
+# GO_MIN_VERSION is the floor from backend/go.mod, and GO_MAX_MINOR is
+# the major.minor of the Go the pinned golangci-lint was built with.
+# Anything outside that window is ignored and GO_VERSION is installed
+# instead. GO_MAX_MINOR is therefore coupled to GOLANGCI_LINT_VERSION
+# below and must be revisited whenever that pin moves; `golangci-lint
+# version` prints the "built with goX.Y.Z" it needs.
 GO_VERSION="1.25.7"
 GO_MIN_VERSION="1.25.5"
+GO_MAX_MINOR="1.25"
 
 # golangci-lint 2.7.2 (2026-08-09). MUST stay equal to the golangci-lint
 # pinned in Dockerfile.backend (currently commit
@@ -47,12 +86,16 @@ GO_MIN_VERSION="1.25.5"
 # v2.12.2, commit c0d3ddc9cf3faa61a4e378e879ece580256d76e5. When that
 # lands, GOLANGCI_LINT_VERSION and every hash in golangci_lint_sha256()
 # below must be updated to the v2.12.2 release archives in the same
-# commit, or local and CI will disagree.
+# commit, or local and CI will disagree. GO_MAX_MINOR must move with
+# it, to the major.minor that release reports as "built with".
 GOLANGCI_LINT_VERSION="2.7.2"
 
 # Where hash-verified archives are unpacked. Version-scoped, so bumping
 # a pin installs alongside the old copy instead of half-overwriting it.
-# Filled in by main() from script/projectname.
+# Filled in by main() from script/projectname. TOOLCHAIN is also the
+# ownership boundary used by link_bin: a symlink pointing inside it is
+# one this script created and may replace.
+TOOLCHAIN=""
 GO_DIR=""
 GOLANGCI_LINT_DIR=""
 
@@ -170,33 +213,67 @@ ver_ge() {
         }'
 }
 
-# ensure_bin_dir: pick the directory provisioned tools are linked into.
+# ensure_bin_dir: the directory provisioned tools are linked into. It is
-# /usr/local/bin when writable (root, or a Homebrew prefix), otherwise
+# always ~/.local/bin: per-user, never a system-wide or package-manager
-# ~/.local/bin, which is prepended to PATH for the rest of this run and
+# prefix. It is put at the front of PATH for the rest of this run, and
-# reported so the user can add it permanently.
+# reported if the caller's own PATH did not already contain it.
 ensure_bin_dir() {
     [ -n "$BIN_DIR" ] && return 0
-    if [ -d /usr/local/bin ] && [ -w /usr/local/bin ]; then
-        BIN_DIR="/usr/local/bin"
-    else
     BIN_DIR="$HOME/.local/bin"
     mkdir -p "$BIN_DIR"
-    fi
+    case "$PATH" in
-    case ":$PATH:" in
+        "$BIN_DIR":*) ;;
-        *":$BIN_DIR:"*) ;;
         *)
             PATH="$BIN_DIR:$PATH"
             export PATH
-            echo "bootstrap: add $BIN_DIR to your PATH" >&2
+            ;;
+    esac
+    case ":$ORIG_PATH:" in
+        *":$BIN_DIR:"*) ;;
+        *)
+            echo "bootstrap: add $BIN_DIR to the front of your PATH, e.g." >&2
+            echo "  export PATH=\"\$HOME/.local/bin:\$PATH\"" >&2
             ;;
     esac
 }
 
+# owned_path <path>: true when <path> lies inside a directory this
+# script provisions, i.e. a link to it is one this script created and
+# may replace. Everything else belongs to the user or to a package
+# manager and is never touched.
+owned_path() {
+    case "$1" in
+        "$TOOLCHAIN"/*) return 0 ;;
+        "$HOME"/.nvm/*) return 0 ;;
+        *) return 1 ;;
+    esac
+}
+
+# refuse_clobber <path> <what>: report that <path> is not ours and stop.
+refuse_clobber() {
+    echo "bootstrap: $1 already exists and $2." >&2
+    echo "  Refusing to replace something this script did not create." >&2
+    echo "  Remove or rename it and re-run bootstrap." >&2
+    exit 1
+}
+
 # link_bin <target> <name>: idempotently expose one provisioned binary
-# on PATH.
+# on PATH. Only an existing symlink into one of our own directories is
+# replaced; a regular file, a directory, or a symlink pointing anywhere
+# else is left intact and bootstrap fails.
 link_bin() {
     ensure_bin_dir
-    ln -sfn "$1" "$BIN_DIR/$2"
+    link="$BIN_DIR/$2"
+    if [ -L "$link" ]; then
+        existing="$(readlink "$link")"
+        if ! owned_path "$existing"; then
+            refuse_clobber "$link" \
+                "is a symlink to $existing, outside this repo's toolchain"
+        fi
+    elif [ -e "$link" ]; then
+        refuse_clobber "$link" "is not a symlink"
+    fi
+    ln -sfn "$1" "$link"
 }
 
 # nvm is a bash script; run a command in a bash with nvm loaded
@@ -229,19 +306,31 @@ ensure_node() {
     done
 }
 
+# ensure_yarn: yarn comes from corepack. Left to itself, `corepack
+# enable` writes its shims next to the corepack binary it resolved, and
+# it writes four of them (yarn, yarnpkg, pnpm, pnpx), not the one asked
+# for. --install-directory keeps all four inside this repo's own
+# toolchain directory, and only yarn is then linked onto PATH. The
+# no-corepack fallback likewise installs into a per-user npm prefix
+# under the toolchain directory instead of npm's global one. Nothing
+# here writes outside $HOME.
 ensure_yarn() {
     if ! missing yarn; then return 0; fi
+    yarn_bin="$TOOLCHAIN/corepack-shims"
+    mkdir -p "$yarn_bin"
     if ! missing corepack; then
-        corepack enable
+        corepack enable --install-directory "$yarn_bin"
         corepack prepare "yarn@$YARN_VERSION" --activate
     elif [ -s "$HOME/.nvm/nvm.sh" ]; then
-        nvm_sh "nvm use $NODE_VERSION >/dev/null && corepack enable && \
+        nvm_sh "nvm use $NODE_VERSION >/dev/null && \
+            corepack enable --install-directory \"$yarn_bin\" && \
             corepack prepare yarn@$YARN_VERSION --activate"
     else
-        npm install -g "yarn@$YARN_VERSION"
+        yarn_bin="$TOOLCHAIN/npm-global/bin"
+        npm install -g --prefix "$TOOLCHAIN/npm-global" "yarn@$YARN_VERSION"
     fi
-    if [ -n "$NODE_BIN" ] && [ -e "$NODE_BIN/yarn" ]; then
+    if [ -e "$yarn_bin/yarn" ]; then
-        link_bin "$NODE_BIN/yarn" yarn
+        link_bin "$yarn_bin/yarn" yarn
     fi
 }
 
@@ -278,14 +367,18 @@ go_sha256() {
     esac
 }
 
-# go_ok: an already-installed go is acceptable if it is at least
+# go_ok: an already-installed go is acceptable only inside the window
-# GO_MIN_VERSION, mirroring how node is used when already present.
+# described at GO_MAX_MINOR: at least GO_MIN_VERSION, and no newer in
+# major.minor than the Go the pinned golangci-lint was built with. A
+# newer host Go is not "good enough", it makes `make check` panic, so it
+# is treated exactly like a missing one.
 go_ok() {
     if missing go; then return 1; fi
     have="$(go version 2>/dev/null | awk '{print $3}')"
     have="${have#go}"
     [ -n "$have" ] || return 1
-    ver_ge "$have" "$GO_MIN_VERSION"
+    ver_ge "$have" "$GO_MIN_VERSION" || return 1
+    ver_ge "$GO_MAX_MINOR" "$(echo "$have" | cut -d. -f1,2)"
 }
 
 ensure_go() {
@@ -371,18 +464,62 @@ ensure_golangci_lint() {
         rm -rf "$tmp"
     fi
     link_bin "$GOLANGCI_LINT_DIR/golangci-lint" golangci-lint
-    if ! golangci_lint_ok; then
+}
-        echo "bootstrap: a different golangci-lint precedes $BIN_DIR on your" >&2
+
-        echo "  PATH; local lint findings may not match what CI gates on" >&2
+# verify_toolchain: bootstrap must not exit 0 while the tools the gate
+# will actually run are not the provisioned ones. Everything above only
+# guarantees the right tools exist and are linked into $BIN_DIR; if
+# something earlier on the caller's PATH shadows them, `make check` --
+# and the pre-commit hook script/setup installs -- still break, and a
+# warning buried in a long bootstrap log is not enough. So the checks
+# re-run against the PATH the caller will have (theirs, plus $BIN_DIR at
+# the front if bootstrap had to ask for it), and a failure is fatal.
+verify_toolchain() {
+    verify_path="$ORIG_PATH"
+    if [ -n "$BIN_DIR" ]; then
+        case ":$ORIG_PATH:" in
+            *":$BIN_DIR:"*) ;;
+            *) verify_path="$BIN_DIR:$ORIG_PATH" ;;
+        esac
     fi
+
+    saved_path="$PATH"
+    PATH="$verify_path"
+    export PATH
+    bad=""
+    go_ok || bad="$bad go"
+    golangci_lint_ok || bad="$bad golangci-lint"
+    for t in gofmt node yarn; do
+        if missing "$t"; then bad="$bad $t"; fi
+    done
+    PATH="$saved_path"
+    export PATH
+
+    [ -z "$bad" ] && return 0
+
+    echo "bootstrap: the toolchain on your PATH cannot run the gate." >&2
+    for t in $bad; do
+        where="$(
+            export PATH="$verify_path"
+            command -v "$t" || echo "not found"
+        )"
+        echo "  $t: $where" >&2
+    done
+    echo "  Expected these to come from $BIN_DIR. Something earlier on" >&2
+    echo "  your PATH is shadowing them, or PATH does not reach it." >&2
+    echo "  Put $BIN_DIR first in PATH, or remove the conflicting tool," >&2
+    echo "  then re-run bootstrap. Failing rather than leaving you a" >&2
+    echo "  bootstrap that reports success and a \`make check\` that does" >&2
+    echo "  not run." >&2
+    exit 1
 }
 
 main() {
     cd "$ROOT"
 
-    toolchain="$HOME/.local/share/$("$ROOT/script/projectname")/toolchain"
+    TOOLCHAIN="$HOME/.local/share/$("$ROOT/script/projectname")/toolchain"
-    GO_DIR="$toolchain/go-$GO_VERSION"
+    GO_DIR="$TOOLCHAIN/go-$GO_VERSION"
-    GOLANGCI_LINT_DIR="$toolchain/golangci-lint-$GOLANGCI_LINT_VERSION"
+    GOLANGCI_LINT_DIR="$TOOLCHAIN/golangci-lint-$GOLANGCI_LINT_VERSION"
 
     if missing make; then pkg_install gnumake make make make; fi
     if missing git; then pkg_install git git git git; fi
@@ -394,6 +531,8 @@ main() {
     ensure_go
     ensure_golangci_lint
 
+    verify_toolchain
+
     echo "bootstrap complete"
 }