Implement RocksDbDirectory object mapping and atomic publication

[kylebernhardy]

Outcome

Map Tantivy logical files to caller-owned RocksDB column-family keys without changing Tantivy segment formats.

Acceptance criteria

• Working files use prefix-stable bounded chunks and become immutable when sealed.
• File handles freeze object identity and visible length so an existing handle never observes changed bytes.
• Logical bindings, sealed object metadata, meta.json, and .managed.json preserve Tantivy semantics.
• Repeated writer flush advances visible length atomically while allowing continued appends.
• atomic_write publishes metadata without exposing a partially reachable generation.
• Reads, writes, deletes, and scans use the Phase 0 approved lease capabilities with no per-I/O JavaScript callback.
• Key encoding and format versions are documented and fixture-tested.

Comment generated by kAIle (Codex GPT-5)