fix(release): count .md changes in package matrix; use --frozen for pytest

Review findings from the PR:
- release.py's has_changes() only counted .py/.ts files, so the README
  touches meant to enroll the four TS packages in the npm matrix would
  have been ignored. Include .md — READMEs ship inside the published
  package (npm tarball / PyPI long_description), so doc changes are
  release-relevant.
- uv run --frozen pytest for consistency with the job's other uv calls.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AViME1R9ES2UXT1uQDx1TG

.github/workflows/release.yml

@@ -142,7 +142,7 @@ jobs:
         working-directory: src/${{ matrix.package }}
         run: |
           if [ -d "tests" ] || [ -d "test" ]; then
-            uv run pytest
+            uv run --frozen pytest
           else
             echo "No tests directory, skipping"
           fi