Fail the Docker build when the lint stage's linter is not the pin

The reordered COPY --from=lint did not make the two stages provably one
toolchain, as the Dockerfile comment, the previous commit message and
TODO.md all claimed. script/bootstrap compares its pin against whatever
PATH resolves, and $GOPATH/bin sits ahead of /usr/local/bin, so any
drift was absorbed: bootstrap rebuilt the pinned version from source,
verified that, and the build went green with the lint stage having
linted at one version and make check having run at another. Bumping the
lint stage image without touching the pin was enough to produce it.

New script/verify-linter-pin fails, naming both versions, unless a given
golangci-lint binary is exactly the version script/bootstrap pins. The
build stage runs it on the binary copied out of the lint stage,
immediately after the copy and before bootstrap, so no reinstall can
satisfy it. The pin is read out of script/bootstrap, which stays its
single source of truth; a pin that cannot be read is a hard failure
rather than a skip. The check takes no CHECK_EPOCH because its only
inputs are the copied binary and script/, so Docker invalidates the
layer exactly when a cached result would stop being true.

The linter version is pinned independently in the lint stage's image
digest and in GOLANGCI_LINT_VERSION, with nothing keeping them in sync;
a half-applied bump is now a build failure instead of a silent split.

ENV PATH keeps $GOPATH/bin, but its comment no longer claims a reinstall
is the reason: bootstrap must be able to run and verify what it
installs, and nothing in this image is shadowed by the entry.

Verified: with the lint stage's linter faked to 2.11.0 after the gates
had really run, the build fails at verify-linter-pin naming 2.11.0 and
2.12.2, with bootstrap and the check gate never reached; an unmodified
make docker is green with all three gates run on a fresh epoch and real
test results. A planted unused finding still fails at the lint stage
with gate check absent from the log; the image still fails
TestScanHardlinkRunFailsTogether under --user 0:0 and passes as uid
1000, both with the Go test cache disabled; and a second build serves
bootstrap, the verify layer and the dependency layers CACHED while the
gates go cold.

TODO.md

@@ -40,18 +40,36 @@
   is gone. `COPY --from=lint /usr/bin/golangci-lint` stays, and moves
   above the bootstrap layer. It is the only edge making this stage
   depend on the lint stage, so deleting it as redundant would end
-  fail-fast linting silently; putting it first also means bootstrap's
-  version check now compares the lint stage's linter against the pin
-  on every build, which is what makes the two stages provably one
-  toolchain instead of two that happen to agree. Letting bootstrap
-  install its own linter here would have reintroduced the second
-  toolchain and paid for a from-source build of it. `$GOPATH/bin`
-  joins `PATH` so that if the copied binary ever stops matching the
-  pin, bootstrap's reinstall lands somewhere `PATH` resolves rather
-  than failing its own verification. Everything added sits above
-  `ARG CHECK_EPOCH`, and the `chown` and `USER builder` still precede
-  `make check`. Verified: bootstrap runs clean under Alpine's `sh` and
-  its `apk` branch, installing `git` and `make` and finding the copied
+  fail-fast linting silently. Letting bootstrap install its own linter
+  here would have reintroduced the second toolchain and paid for a
+  from-source build of it. What makes the two stages provably one
+  toolchain rather than two that happen to agree is a new
+  `script/verify-linter-pin`, run in the build stage on the binary
+  that arrives from the lint stage, before bootstrap: it fails the
+  build naming both versions unless that binary is the version
+  `script/bootstrap` pins. Bootstrap's own check could not serve that
+  purpose — it reinstalls its pin from source and then verifies
+  whatever `PATH` resolves, so drift self-heals silently and a lint
+  stage image bumped on its own would lint at the new version while
+  `make check` ran at the old one, green. The linter version is pinned
+  in two independent places (the lint stage image digest and
+  `GOLANGCI_LINT_VERSION`) and nothing else keeps them in sync, so a
+  half-applied bump is now a build failure. The pin is read out of
+  `script/bootstrap`, which stays the single source of truth; a pin
+  that cannot be read is a hard failure, not a skip. The check needs
+  no `CHECK_EPOCH`: its only inputs are the copied binary and
+  `script/`, so Docker invalidates the layer exactly when a cached
+  result would stop being true, and it is documented with the other
+  entrypoints in the README. `$GOPATH/bin` joins `PATH` because
+  that is where bootstrap's `go install` lands and bootstrap verifies
+  its installs against what `PATH` resolves — nothing in the image is
+  shadowed by it, the directory does not exist until bootstrap runs.
+  Everything added sits above `ARG CHECK_EPOCH`, and the `chown` and
+  `USER builder` still precede `make check`. Verified: the guard fails
+  the build with both versions named when the lint stage's linter is
+  faked to a different version, and an unmodified build still passes
+  it; bootstrap runs clean under Alpine's `sh` and its `apk` branch,
+  installing `git` and `make` and finding the copied
   linter already at the pin; a second build served the bootstrap and
   dependency layers `CACHED` while both gates ran with a fresh epoch;
   a planted `unused` finding failed the build at the lint gate in