Separate structural vs value update channels

[FuJacob]

Context

Currently all WebSocket events flow through the same path with identical handling. Sim separates two distinct channels with different reliability and performance characteristics:

1. Structural channel (pessimistic) — node create/delete, edge create/delete. Persist first, then broadcast.
2. Value channel (optimistic) — text edits, subblock updates. Broadcast immediately, persist with server-side coalescing/debounce.

Position updates (node move/drag) are a special case: broadcast immediately for smooth collaborative dragging, persist on drag-end.

What to build

Backend: Two processing paths

• Structural handler: Validates → persists to DB → broadcasts to peers → sends ack to sender
• Value handler: Validates → broadcasts to peers → coalesces writes → persists in batch (25ms window)
• Position handler: Broadcasts immediately → persists on node_drag_end

Frontend: Channel-aware send

• Tag outbound events with their channel type, or route by event type
• Structural ops wait for ack before marking complete in operation queue
• Value ops are optimistic (no ack wait)

Server-side coalescing

• For value updates targeting the same field within a coalescing window (~25ms), only persist the final value
• Reduces DB writes from hundreds to dozens for a typical typing session

Current code references

• services/collab/src/main/java/com/leetdoodle/collab/handler/CanvasWebSocketHandler.java — handleCanvasEvent() treats all events identically
• frontend/src/shared/events.ts — event type definitions (no channel distinction today)

Acceptance criteria

• Structural ops persist before broadcast (pessimistic)
• Value ops broadcast before persist (optimistic)
• Position updates broadcast immediately, persist on drag-end
• Server coalesces rapid value updates to reduce DB writes
• Event types are categorized into channels