feat: SqliteStore — persistent verbatim storage via better-sqlite3

[SimplyLiz]

Summary

• Adds SqliteStore class for persisting CompressResult data (messages + verbatim) in SQLite
• better-sqlite3 declared as optional peer dependency — library stays zero-dep for users who don't need persistence
• Atomic save via SQLite transactions satisfying the verbatim atomicity requirement
• Lazy lookup() returns a StoreLookup function for uncompress() — single-row SELECTs, no full verbatim load
• Verbatim entries accumulate across save() calls so recursive uncompress() works across compression rounds
• 13 tests covering roundtrip, isolation, multi-round accumulation, recursive expansion, extra fields, and cleanup

API

const store = await SqliteStore.open('data.db'); // or ':memory:'
store.save('conv-1', compressResult);
const data = store.load('conv-1');
const restored = uncompress(data.messages, store.lookup('conv-1'));
store.delete('conv-1');
store.list(); // ['conv-2', ...]
store.close();

Test plan

• npm test — 684 tests pass (13 new)
• npm run build — clean
• npm run lint && npm run format:check — clean