[Tinker] Add deterministic SQLite hot-path contention repro

[j316chuck]

Summary

• Add a deterministic SQLite contention repro only; no SkyRL production code changes.
• Exercise four waves of 512 concurrent samples and training-future submissions, plus heartbeats and future retrievals, against a 5+10 SQLite connection pool.
• Assert all 2,048 EXTERNAL futures, all 2,048 training futures, and all 128 heartbeats persist exactly.

Dependency

This stacks on upstream PR NovaSky-AI#2127 and is intended to land after it.

Testing

• uv run --extra dev --extra tinker pytest tests/tinker/test_sqlite_hot_path_repro.py -q — 1 passed
• uv run --extra dev pre-commit run --files tests/tinker/test_sqlite_hot_path_repro.py — passed