ci: optimize test workflows — reduce jobs, share venv via artifact

- Restructure tests.yml: install once per Python version, share .venv
  via artifact instead of 32 independent installs
- Reduce test groups from 8 to 4 (tests only take ~60s per group)
- Only test Python 3.12+3.13 on PRs; full matrix on push to main
- Switch all workflows from manual actions/cache to setup-uv built-in
  caching, eliminating cache race conditions
- Add --frozen flag to uv sync for deterministic CI installs
- Re-enable duration-based test splitting with least_duration algorithm
  (was disabled due to a bug in the path filter)
- Fix update-test-durations path filter (tests/**/*.py never matched
  actual test dirs under lib/)
- Add concurrency group with cancel-in-progress for PR runs
- Add gate jobs to satisfy existing branch protection required checks

.github/workflows/build-uv-cache.yml

@@ -25,24 +25,12 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
-      - name: Install uv
+      - name: Install uv and populate cache
         uses: astral-sh/setup-uv@v6
         with:
           version: "0.8.4"
           python-version: ${{ matrix.python-version }}
-          enable-cache: false
+          enable-cache: true
 
-      - name: Install dependencies and populate cache
-        run: |
-          echo "Building global UV cache for Python ${{ matrix.python-version }}..."
-          uv sync --all-groups --all-extras --no-install-project
-          echo "Cache populated successfully"
-
-      - name: Save uv caches
-        uses: actions/cache/save@v4
-        with:
-          path: |
-            ~/.cache/uv
-            ~/.local/share/uv
-            .venv
-          key: uv-main-py${{ matrix.python-version }}-${{ hashFiles('uv.lock') }}
+      - name: Install dependencies
+        run: uv sync --all-groups --all-extras --frozen --no-install-project