Collab Phase 3: Draft Buffer API and Operation Intake for Deferred Persistence

[cubap]

Summary

Add transient draft operation intake and storage so interfaces can submit collaborative edits without immediate durable RERUM writes.

Scope

• Add draft operation intake endpoint(s) for line text and bounds patches.
• Store coalesced transient draft state by project/page/line/user.
• Emit op accepted or rejected acknowledgements.
• Fanout accepted operations to websocket room members.
• Add validation and limits for payload size and queue depth.

Acceptance Criteria

• Draft operations are accepted and visible to collaborators in realtime.
• Duplicate retry submissions are deduplicated by idempotency key.
• No durable RERUM write occurs in this phase.
• Rejected operations return machine-readable reason.

Dependency

• Depends on websocket skeleton issue.