Mark superseded commits honestly instead of skipped (closes #152)

Gitea cancels an in-flight run when a newer commit lands on the same
branch and records the cancellation as `failure` / "Has been
cancelled". The workflow rewrote that to `skipped`, but Gitea's
Combine() folds `skipped` into `success`, so the combined-status API
returned green for a commit nothing had ever tested. Rewrite it to
`failure` / "Superseded by a newer commit; never tested" instead:
red-but-honest, and never `pending`, which would block the commit
forever.

Re-running the superseded commit would have been better still, but is
not reachable on this Gitea (1.25.4): its API exposes no rerun
endpoint, workflow dispatch takes a ref rather than a SHA, and every
replay would be a full uncached build with no bound on how many pile
up behind a burst of merges.

The step also stops hardcoding its status context: the logic moves into
script/ci-mark-superseded, which derives the context from the workflow
name, job name and event -- the same three values Gitea builds it from
-- and fails loudly when no status on the commit being built carries
that context, so renaming the workflow or the job cannot silently
disable the rewrite. That is item 2 of
#147; item 1 there is
untouched.

Tests drive the script against a fake Gitea covering the cancelled,
laundered-skipped, genuinely-failed, passing and renamed cases, so jq
joins the builder image to run them.

.gitea/workflows/check.yml

@@ -16,36 +16,15 @@ jobs:
           # that touched the Docker build context.
           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-      - name: Neutralize superseded run statuses
+      - name: Mark superseded run statuses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