feat: group commit mode for ~12x faster writes — v0.5.0

[mbocevski]

Summary

writeMode: 'group' buffers operations in memory and flushes as a single disk write. ~12x faster than immediate mode for sustained writes.

Configuration

await store.open(dir, {
  writeMode: 'group',      // default: 'immediate'
  groupCommitSize: 50,     // flush when buffer reaches N ops
  groupCommitMs: 100,      // flush after N ms idle
});

Safety

• Forced to immediate when agentId is set (multi-writer). Logs warning.
• Buffer flushed on close(), compact(), and explicit flush().
• Timer ensures buffer is written within groupCommitMs even with few ops.

Benchmark

500 individual writes: immediate=12ms, group=1ms, 12.4x speedup.

Test plan

175 tests (was 168). 7 new group commit tests:

• Buffer flushes on size threshold
• Buffer flushes on timer
• close() flushes buffer
• Explicit flush() works
• Undo works with group commit
• Forced to immediate with agentId
• Performance comparison (12.4x speedup verified)