Mark superseded commits honestly instead of skipped (closes #152)

Gitea records a cancelled run as failure, and the #119 repair rewrote
that to skipped. Gitea's Combine() folds skipped into success, so a
commit nothing ever tested reported a combined green — observed on three
commits on next, including the very change a prior integration review
had failed a PR for.

Superseded commits are now marked failure with an honest description, so
never-tested no longer reads as passed and git bisect archaeology can
tell "passed", "failed" and "never ran" apart. Option 1, re-running the
superseded commit, was verified unreachable for automation on Gitea
1.25.4: no rerun endpoint, dispatches takes a ref not a SHA and lands
under a different context, and CancelPreviousJobs is unconditional.

The rewrite moves out of the workflow into script/ci-mark-superseded so
the tested artifact is the shipped one, and every failure path in it is
loud: an unparseable or empty ANCESTOR_LIMIT, an unreadable ancestor
status, and a shallow clone all abort rather than exiting 0 having
marked nothing. Each has a regression test. The status context is
derived rather than hardcoded, which also closes #147 item 2; item 1
remains open.

Independently reviewed four times. The final reviewer confirmed the
shallow-clone test is genuinely shallow — a file:// URL is load-bearing,
since git silently ignores --depth on a local path — and that deleting
the guard fails that one test out of 331 and cannot pass for the wrong
reason. They also reproduced deterministically that go test's cache
serves a stale PASS after a script-only edit, which internal/ciscript's
doc.go now records.

.gitea/workflows/check.yml

@@ -13,39 +13,19 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2 2024-10-23
         with:
           # The fingerprint step below needs history to find the last commit
-          # that touched the Docker build context.
+          # that touched the Docker build context, and the superseded-status
+          # step needs it to walk ancestors (it aborts on a shallow clone).
           fetch-depth: 0
 
-      - name: Neutralize superseded run statuses
+      - name: Mark superseded run statuses
         # Gitea cancels the in-flight run when another commit is pushed to the
         # same branch and records the cancellation as `failure`, so a commit
-        # that was never tested reads red. The cancellation is unconditional
-        # server-side for push events and cannot be disabled from a workflow
-        # file, so the superseding run rewrites those statuses to `skipped`.
-        # Only the exact cancellation status is touched; a real failure is
-        # left alone.
+        # that was never tested reads as a test result. The script rewrites
+        # those statuses to say what happened. See its header for why the
+        # state stays `failure` and not `skipped`.
         env:
           GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
-        run: |
-          set -eu
-          api="${GITHUB_API_URL}/repos/${GITHUB_REPOSITORY}"
-          ctx='check / check (push)'
-          for sha in $(git rev-list --max-count=20 "${GITHUB_SHA}^" || true); do
-            latest="$(curl -sf "${api}/commits/${sha}/status" | jq -r \
-              --arg c "$ctx" \
-              '[.statuses[] | select(.context == $c)][0] // empty
-               | "\(.status)|\(.description)"')" || continue
-            [ "$latest" = 'failure|Has been cancelled' ] || continue
-            curl -sf -X POST "${api}/statuses/${sha}" \
-              -H "Authorization: token ${GITEA_TOKEN}" \
-              -H 'Content-Type: application/json' \
-              -d "$(jq -nc --arg c "$ctx" '{
-                context: $c,
-                state: "skipped",
-                description: "Superseded by a newer commit; never tested"
-              }')" >/dev/null
-            echo "neutralized superseded status on ${sha}"
-          done
+        run: script/ci-mark-superseded
 
       - name: Fingerprint the build context
         # `.dockerignore` keeps docs out of the build context, so a docs-only