Run the merge gate only for changes its tests read

.github/workflows/test.yml

@@ -16,12 +16,28 @@ jobs:
       - uses: actions/checkout@v6
         with:
           persist-credentials: false
+          fetch-depth: 0
+      - name: Relevant changes
+        id: changes
+        run: |
+          if [ "$GITHUB_EVENT_NAME" = "pull_request" ]; then
+            git fetch --no-tags origin "$GITHUB_BASE_REF"
+            base="origin/$GITHUB_BASE_REF"
+          else
+            base="HEAD^"
+          fi
+          if git diff --quiet "$base"...HEAD -- '*.go' go.mod go.sum Dockerfile .dockerignore .github data README.md; then
+            echo "code=false" >> "$GITHUB_OUTPUT"
+          else
+            echo "code=true" >> "$GITHUB_OUTPUT"
+          fi
       # `docker build` streams the context to the daemon. A compose bind-mount of
       # `.` mounts an empty host dir instead, because the job is itself a container.
       - name: Latest supported Go
+        if: ${{ steps.changes.outputs.code == 'true' }}
         run: docker build .
       # Runs even when the step above failed, so a red build says whether the
       # failure is version-specific.
       - name: Lowest supported Go
-        if: ${{ !cancelled() }}
+        if: ${{ !cancelled() && steps.changes.outputs.code == 'true' }}
         run: docker build --build-arg GO_VERSION=1.22.12 .

README.md

@@ -460,6 +460,13 @@ tokens add cost in proportion to the output.
   almost always costs an allocation too (a lost pre-size, a per-item map, an extra
   copy). The benchmark suite (see Development) reports time for a human, not as a
   pass/fail gate.
+- **The gate runs only when something it consumes changes.** A PR that touches
+  none of the Go and module files, the Dockerfile, `.dockerignore`, the
+  workflow, the shipped data nor the README skips `docker build` and passes
+  as-is — those are what the build feeds or the tests read (`data_test.go` and
+  `loading_test.go` load `data/`, `readme_test.go` renders every README
+  example), so any other change is a verdict already decided, and waiting would
+  be idle. The check still reports success, so the merge gate stays whole.
 
 ## Development
 
@@ -485,7 +492,10 @@ docker compose run --rm --user "$(id -u):$(id -g)" tidy  # go mod tidy
 
 Every pull request runs `docker build .` against both the latest and the lowest
 supported Go, and must pass before it can be merged. That build is the whole
-gate — vet, complexity, format check and tests — so run it locally before pushing:
+gate — vet, complexity, format check and tests — so run it locally before pushing.
+A PR that touches none of the Go and module files, the Dockerfile,
+`.dockerignore`, the workflow, the shipped data nor the README skips the build
+and passes as-is (see [Decisions](#decisions)):
 
 ```sh
 docker build .                                  # latest