ci: shard the test suite across parallel jobs

[apresmoi]

Why

CI runs ~20 min. Root cause: ubuntu-latest is a 2-core runner, and node --test parallelizes at the file level — so the CPU-heavy dynamics build / DynamicsSession suites can't spread out and run nearly serially. (The same suite is ~2 min on an 18-core dev box.)

What

• Split the single job into a fast check job (npm audit + typecheck) and a test job.
• Shard test across 4 parallel jobs via node --test's built-in --test-shard=i/4, so wall time is the slowest shard, not the whole suite.

Expected: ~20 min → ~5 min, no test changes.

Notes

• --test-shard splits by file count, not duration, so the heavy suites may not be perfectly balanced; bump the shard count (or hand-group the heavy dynamics build suites) if one shard stays slow.
• The durable follow-up is to make those suites build the dynamics artifact once and reuse it across cases instead of rebuilding per test — that lowers the floor regardless of runner size.