Epic: Implement Tantivy storage through Harper's existing RocksDB APIs

[kylebernhardy]

Outcome

Build the Harper-backed Tantivy Directory using supported Harper storage APIs and Harper's existing RocksDB database.

Acceptance criteria

• Start with the real Harper vertical slice in issue Phase 0: prove the Harper-backed Tantivy storage and derived-index path #7; no dependency on rocksdb-js native-lease issues or PR.
• Separate source delivery/replay from binary storage, and reuse the shared engine and Directory conformance harness.
• Prove bounded native-to-host storage transport, legal JS thread access, buffer ownership and teardown without deadlocks.
• Implement immutable object mapping, atomic metadata publication, exact durability ordering, crash recovery, orphan cleanup and generation lifecycle.
• Integrate Harper backup, close, drop and recreation; no second DB opener, private patched rocksdb-js or native-file fallback.
• Qualify multi-index progress alongside foreground Harper traffic; profile measured bottlenecks before adding optimization infrastructure.

Design

Tantivy storage through Harper. The Harper integration is planned; the native backend is implemented. No production change to base rocksdb-js is required by this work.

Comment generated by kAIle (GPT-6)