Walk the path CI runs in lint-once, and pin the branch the container installs (closes #33)

The header of test/packaging/lint-once.test.ts claimed a duplicate prettier
pass is caught wherever it is added. It was not: the walk started at
`make check`, which never reads `Dockerfile`, so appending
`RUN yarn run prettier --check .` to the image that `script/cibuild` builds
left the suite green — two prettier passes on the one path where it matters
most. The walk now also starts at `.gitea/workflows/check.yml` and follows
its `run:` steps into `script/cibuild` and from there into both images, so
the graph under test is the one CI executes rather than the one it was
assumed to execute. Reaching `script/cibuild` and `Dockerfile` is asserted,
and the test and build image is asserted to invoke prettier zero times.

The lockfile assertion was a substring check against the whole of
`script/bootstrap`. That script has two install sites, and the containers
take the second, because the pinned node image ships yarn; changing that
site to a bare `yarn install` kept the suite green while the container's
install stopped being pinned. `install_js_deps` is now resolved out of the
script and split at its `missing yarn` guard, and every `yarn install`
occurrence in each branch is required to carry `--frozen-lockfile`. That the
container runs `script/bootstrap` at all is asserted too, so the lockfile
assertions cannot end up describing a script the image never executes.

Prettier is counted per occurrence instead of per line:
`prettier --check . && prettier --check src` was one invocation by the old
count. The `continue` that followed a counted line also dropped every
script, make, yarn and docker edge sharing that line, so a subtree could be
hidden behind a single `&&`; edges are now extracted from every line.

Undercounting is what would make this file worthless, so every way of
reaching nothing is a thrown error rather than a quiet zero: an unknown
Makefile target, an unknown package.json script, a missing script file, a
node that resolves to no commands, and an unknown node kind. All five are
tested, as is a walk that legitimately counts zero, and the cycle guard.

Every assertion in the file was mutation-tested: changed to assert something
else, run, and confirmed to fail for its own named reason. The two mutations
above were reproduced and both now turn the suite red.

test/packaging/entrypoints.test.ts said `make check` runs test, lint and
fmt-check. Formatting has been part of the lint container since the
duplicate host pass was removed, so the comment now says what it does.

TODO.md

@@ -18,6 +18,22 @@ Update the README API reference section to match the current implementation.
 
 # Completed Steps
 
+- 2026-08-10: Made `lint-once.test.ts` enforce what its header claims. It walked
+  `make check` only, so it never read `Dockerfile` — the image CI builds through
+  `script/cibuild` — and a second `prettier --check .` could be added there with
+  the suite staying green. The walk now also starts at
+  `.gitea/workflows/check.yml` and follows its `run:` steps, so the graph under
+  test is the one CI executes rather than the one someone assumed it executes.
+  The lockfile assertion was a substring check against the whole of
+  `script/bootstrap`, which has two install sites and so reported the branch the
+  containers never take; the two branches are now resolved separately and every
+  `yarn install` in each is required to be `--frozen-lockfile`. Prettier is
+  counted per occurrence instead of per line, so two invocations chained with
+  `&&` no longer read as one, and edges are followed on counted lines instead of
+  being skipped. Every way for the walk to reach nothing — an unknown target, an
+  unknown script, a missing file, a node with no commands, an unknown node kind
+  — is a thrown error rather than a quiet zero. Every assertion in the file was
+  mutation-tested individually.
 - 2026-08-10: Stopped `make check` running `prettier --check .` twice. Since
   linting moved into Docker, the duplicate was one container pass and one host
   pass of the same check: `script/lint` builds `Dockerfile.lint`, which runs