Implement MmapDirectory mode as the standalone backend and reference oracle

[kylebernhardy]

Outcome

Support Tantivy native filesystem storage outside Harper and use it as the behavioral and performance reference for the Rocks adapter.

Acceptance criteria

• The native factory delegates storage and locking to Tantivy MmapDirectory.
• Native mode implements the same public schema, mutation, search, lifecycle, status, and metrics contracts as Rocks mode.
• Filesystem paths are accepted only by the native entry point.
• Harper cannot select or fall back to native mode.
• Paired conformance and benchmark fixtures can run the same workload against both backends.

Comment generated by kAIle (Codex GPT-5)