Parallel Test Execution

[kubrickcode]

Parallel test execution and CI sharding.

# .github/workflows/test.yml
jobs:
  test:
    strategy:
      matrix:
        shard: [1, 2, 3, 4]
    steps:
      - name: Run tests (Shard ${{ matrix.shard }}/4)
        env:
          JEST_SHARD: '{"current": ${{ matrix.shard }}, "total": 4}'