Implement mutation batching, commit publication, reader reload, and status

[kylebernhardy]

Outcome

Provide the indexing lifecycle needed by standalone callers and Harper derived delivery.

Acceptance criteria

• Packed add, replace-by-primary-key, delete, and replay-safe version suppression are supported.
• Batches preserve ordering and report per-batch completion without per-record JavaScript callbacks.
• Commit preparation, payload assignment, publication, rollback, merge-only publication, and reader reload follow Tantivy semantics.
• The commit payload round-trips versioned opaque checkpoint bytes exactly.
• Status reports writer health, searchable generation, queue depth, commit/reload state, and last failure.
• Standalone mode does not invent Harper transaction-log semantics.

Comment generated by kAIle (Codex GPT-5)