Collab Phase 6: Idempotent Overwrite and Conflict Payload Enhancements for Deferred Commit Clients

## Summary
Enhance RERUM durability APIs for deferred commit workloads with idempotency and richer conflict metadata.

## Scope
- Support idempotency keys for overwrite and retry safety.
- Ensure conflict responses include current revision and current document metadata.
- Evaluate optional bulk overwrite endpoint for batched page flushes.
- Emit metrics for write rate, conflict rate, and latency.

## Acceptance Criteria
- Retries using same idempotency key do not duplicate writes.
- Conflict response contains fields needed for client reconciliation.
- Metrics are emitted for durability observability.

## Dependency
- Aligns with TPEN services durable flush worker issue.

Inizia individuando gli entry point della RERUM durability API e il flusso del client per il deferred commit, quindi esamina la dipendenza dall’issue del durable flush worker dei servizi TPEN. Il lavoro è completato quando i retry con una sola idempotency key non duplicano le scritture, le conflict responses espongono la revisione corrente e i metadati del documento e vengono emesse metriche di scrittura, conflitto e latenza.

Scritto dal modello di indicizzazione a partire dal testo della issue.