Timestamp-based conflict resolution for structural ops

[FuJacob]

Context

Structural operations (node create/move/delete, edge create/delete) currently use simple last-write-wins with no formal ordering. The server relays messages in arrival order without conflict detection. Sim uses client-generated timestamps with deterministic tiebreaking for structural conflict resolution.

Text content already has proper CRDT conflict resolution (RGA). This ticket is about the structural layer only.

What to build

Client-side: Timestamp every operation

• Every outbound structural operation includes a timestamp field (high-resolution client clock, e.g., performance.now() + wall clock)
• Format: ISO 8601 with millisecond precision, or monotonic counter

Server-side: Timestamp-based ordering

• Position conflicts: When two users simultaneously move the same node, persist the one with the latest timestamp. Broadcast both in sequence — clients converge to the latest.
• Create/delete conflicts: If a delete arrives for an entity that was modified after the delete's timestamp, the modification wins (the entity survives). If the delete is newer, it wins.

Deterministic tiebreaking

• When timestamps are identical (within precision), use userId lexicographic ordering as tiebreaker
• Ensures all clients converge to the same state given the same set of ops

Server-side coalescing for position

• Position updates within a ~25ms window for the same node: only persist the latest timestamp
• Reduces DB churn during collaborative dragging

Current code references

• frontend/src/shared/events.ts — outbound event types (no timestamp field today)
• services/collab/src/main/java/com/leetdoodle/collab/handler/CanvasWebSocketHandler.java — relay logic (no conflict detection)

Acceptance criteria

• All structural ops carry client-generated timestamps
• Server resolves position conflicts using latest timestamp
• Deterministic tiebreaker (userId) for equal timestamps
• All clients converge to identical state given same operations
• Server coalesces rapid position updates before persisting