feat(ci): assign coverage partitions by recorded file duration

Replace Vitest's hash-based --shard with a coordinator-side
longest-processing-time assignment. The coordinator collects the
instrumented inventory from a vitest list run (dropping the exempt
heavy suites that list does not exclude), reads per-file durations from
the Vitest results cache, and seeds heavy subprocess-bound suites into
different partitions. A weight-aware test fails when assignment ignores
recorded weights, verified by injecting a file-count-only rule.

Windows coverage measured partition spread of 442s (275-717s) under
hash sharding; a simulation with the same file durations and the new
assignment balances partitions to within 21s, cutting the critical
partition to roughly half.

.github/workflows/ci.yml

@@ -459,6 +459,19 @@ jobs:
       - uses: actions/checkout@v6
         with:
           persist-credentials: false
+      # Checkout removes the gitignored duration history, so restore it from
+      # the GitHub cache before coverage and save the updated one afterwards.
+      # The coordinator then weights partitions by measured durations on the
+      # second run instead of degrading to a file-count split. GitHub cache
+      # entries are immutable, so the save key is unique per run and the
+      # restore matches the newest entry through the stable prefix.
+      - name: Restore coverage duration history
+        uses: actions/cache/restore@v4
+        with:
+          path: .coverage-times.json
+          key: coverage-times-${{ github.run_id }}
+          restore-keys: |
+            coverage-times-
       - name: Enable Developer Mode (symlink support)
         shell: pwsh
         run: >-
@@ -483,6 +496,15 @@ jobs:
       - name: Run Windows coverage
         shell: pwsh
         run: pnpm run check:ci:coverage
+      - name: Save coverage duration history
+        # Coverage flakes must not prevent the cache from building; the
+        # measured durations remain useful even when a partition failed. The
+        # per-run key keeps every save a fresh immutable cache entry.
+        if: always()
+        uses: actions/cache/save@v4
+        with:
+          path: .coverage-times.json
+          key: coverage-times-${{ github.run_id }}
 
   windows-native-tests:
     if: github.event_name == 'pull_request'