Make the CI gate execute the checks it reports on (closes #119)

The workflow writes a build-context fingerprint before calling script/cibuild, so a code commit invalidates the COPY layer of the lint and builder stages and the checks really run, while a docs-only commit still replays from cache. A superseding run also rewrites the exact failure/Has been cancelled status left on commits that were never tested to skipped, so cancellation no longer reads as red. script/cibuild itself is untouched.

README.md

@@ -1109,6 +1109,34 @@ binary is statically linked and runs on Alpine.
 `docker build .` is the CI gate — if it passes, the code is formatted,
 linted, tested, and compiled.
 
+#### CI gate honesty
+
+A layer cache lets `docker build .` exit 0 in seconds with the lint and
+test stages replayed rather than executed, which would make a green
+check meaningless. The `check` workflow therefore writes
+`.ci-fingerprint` into the build context before building. Its value is
+the hash of the last commit that touched the build context, so:
+
+- Any commit that changes code (including a squash merge whose tree
+  matches an already-built branch) gets a new fingerprint, invalidates
+  the `COPY . .` layer of both check stages, and really runs
+  `make fmt-check`, `make lint`, `make test`, and `make build`. A run
+  that reports success ran them.
+- A docs-only commit leaves the fingerprint unchanged — `.dockerignore`
+  excludes `*.md` and `LICENSE` from the context anyway — so the image
+  replays from cache and costs seconds.
+
+The module download layer sits above `COPY . .` and stays cached either
+way.
+
+The workflow's first step covers a second way the gate lied: Gitea
+cancels an in-flight run when a newer commit lands on the same branch
+and records that cancellation as a `failure` status, marking a commit
+red that was never tested. Cancellation is unconditional server-side for
+push events, so the superseding run rewrites the exact
+`Has been cancelled` status to `skipped`. Genuine failures are never
+touched.
+
 ## TODO
 
 See [TODO.md](TODO.md).