fix(ci): do not cancel push-to-main coverage baseline runs

The coverage gate's `concurrency.cancel-in-progress: true` was cancelling
the push-to-main runs that compute and SAVE the baseline cache. A rapid
run of merges cancelled each baseline run before it could save, so the
cache was never populated and every PR's gate stayed "informational - no
baseline" forever.

Only cancel superseded PR runs; let every push-to-main run finish and
save its baseline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WvXMHHbYGddVPpmQR2XmK1

Author: claude

.github/workflows/coverage.yml

@@ -28,7 +28,11 @@ on:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
-  cancel-in-progress: true
+  # Only cancel a SUPERSEDED PR run (a newer commit on the same PR). NEVER
+  # cancel a push-to-main run: those compute and SAVE the baseline cache, and a
+  # rapid run of merges would otherwise cancel each baseline run before it
+  # saved, leaving the gate permanently without a baseline to compare against.
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
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