perf(test): add timing telemetry, cold-start probe, and baseline snapshot

- ci-test-shard.mjs: --write-timings aggregation into scripts/test-timings.json (bucketed, newer-snapshot-protected, corrupt-shard tolerant) and --cold-start-probe; CI shard invocations emit vitest json timings
- test-changed.mjs: structured mode/reason telemetry line (+ --print-mode)
- pr-checks.yml: upload per-shard timing artifacts
- baseline: docs/test-speed-baseline-2026-06-03.md (core 41s, engine 179s, cli 49s; cold-start ~1.3-1.8s/process => U8 gate: worthwhile-not-urgent)

.github/workflows/pr-checks.yml

@@ -85,3 +85,17 @@ jobs:
 
       - name: Test (deterministic shard)
         run: pnpm test:ci:shard --shard ${{ matrix.shard }} --total 4
+
+      # U1 (R4): each shard emits per-file vitest JSON timing reporter output
+      # under .timings/. Upload as an artifact so the timing snapshot can be
+      # refreshed locally/from the default branch via
+      # `node scripts/ci-test-shard.mjs --write-timings`. We do NOT commit the
+      # snapshot from PR branches — refresh is manual/scheduled only.
+      - name: Upload per-shard test timings
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: test-timings-shard-${{ matrix.shard }}
+          path: .timings/timings-*.json
+          if-no-files-found: ignore
+          retention-days: 14