feat: WAL tailing and delta encoding — v0.4.0

[mbocevski]

Summary

• WAL tailing — store.tail() reads new operations from the WAL since the last known position. store.watch(callback, interval) polls and notifies. unwatch() and close() stop polling. Works in readOnly, single-writer, and multi-writer modes. Enables cross-process live updates.
• Delta encoding — update operations automatically use delta encoding when the patch is smaller than the full prev record. Stores {$set, $unset} instead of full previous record. encoding: "delta" field. Undo correctly applies reverse patches.

Both features go through the StorageBackend interface — custom backends (S3, etc.) get them for free.

Test plan

166 tests (was 142). 24 new tests:

• 7 WAL tailing tests: tail empty, tail picks up new ops, incremental tail, watch callback, unwatch, close auto-unwatches
• 17 delta encoding tests: createDelta, applyDelta, isDeltaSmaller, Store integration (auto delta, undo with delta, reopen with delta)
• Existing test updated for delta-aware history check
• Build + lint clean