Add src/db.ts that dynamically imports bun:sqlite under Bun and
better-sqlite3 under Node.js. Exports openDatabase(), loadSqliteVec(),
and a shared Database interface.
- sqlite-vec loading is now optional — FTS works without it, vector
ops throw a clear error if unavailable
- CI tests both runtimes: Node 22/23 via vitest, Bun via bun test
- All 104 unit tests pass on both Node and Bun
dcedfb5268