ci: cancel in-progress runs on same-ref pushes

Add a top-level concurrency block to ci.yml keyed on
(workflow, ref) with cancel-in-progress: true. Pushes (including
force-pushes) to the same ref now retire any in-flight run for
that ref rather than letting the superseded and current-tip runs
both burn runner minutes.

Motivated by a 2026-05-14 force-push experience where two CI runs
ran concurrently against a feature branch — the original push at
c76ec99 continued for ~17 minutes alongside the amended push at
927ef47, despite only the latter being the live tip.

Scope deliberately limited to ci.yml. Tag-triggered release-build
workflows (package-*.yml, aur-publish-*.yml) are untouched — they
operate on per-tag refs that already form distinct concurrency
groups and release artifact builds generally should not be
cancellable by unrelated activity.

.github/workflows/ci.yml

@@ -11,6 +11,10 @@ on:
         type: boolean
         default: false
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 permissions:
   checks: write
   contents: read