Undo/redo system via inverse operation stack

[FuJacob]

Context

There is currently no undo/redo system. Sim implements undo/redo as a thin wrapper over the socket system — undo is just another edit that generates an inverse operation and sends it through the normal operation queue.

What to build

Per-user undo/redo stacks

• Each user maintains a local undo stack and redo stack
• Every operation records its inverse:
  • Add node → inverse: Remove node (snapshot full node state)
  • Remove node → inverse: Add node (restore from snapshot)
  • Move node → inverse: Move node (original position)
  • Update text → inverse: Update text (previous value)
  • Add edge → inverse: Remove edge
  • Remove edge → inverse: Add edge (snapshot edge)

Undo execution

• Cmd+Z / Ctrl+Z: Pop from undo stack → push to redo stack → execute inverse op through the operation queue
• Cmd+Shift+Z / Ctrl+Y: Pop from redo stack → push to undo stack → execute forward op through the operation queue
• Inverse operations flow through the normal WebSocket system — other users see the change in real-time

Coalescing consecutive ops

• Consecutive same-type operations should coalesce in the stack
• e.g., dragging a node through 50 positions → single undo entry (start position → end position)
• Typing in a text field → coalesce by time window or word boundary

Multiplayer-safe stack pruning

• When a remote user deletes an entity, prune any undo entries that reference it
• Example: User A has "add edge to Block X" in undo stack, User B deletes Block X → prune that entry
• Prevents executing inverse ops on entities that no longer exist

Circular recording prevention

• Mark undo/redo operations as isUndo: true during execution
• These operations should NOT be recorded into the undo stack themselves

Current code references

• frontend/src/canvas/model/useCanvasDocument.ts — command functions that would generate undo entries
• frontend/src/canvas/hooks/useCanvasShortcutContainer.ts — keyboard shortcut handling (add Cmd+Z here)

Acceptance criteria

• Cmd+Z undoes last local action
• Cmd+Shift+Z redoes last undone action
• Undo/redo operations broadcast to other users via WebSocket
• Consecutive drag/type operations coalesce into single undo entries
• Stack entries pruned when referenced entities are deleted by remote users
• Undo operations don't create new undo entries