Undo/redo module

[bart-krakowski]

Parent

#25

What to build

Extract undo/redo into a proper module that wraps atomic write batches, not individual field mutations. A single user action (e.g. move event + cascaded dependent shifts) is undone as one step — not as a sequence of individual commits.

Acceptance criteria

• `UndoRedoModule` plugs into the kernel via the module API (Kernel + module registration API #28), hooking the `undo-snapshot/emit` stage of the write pipeline
• `undo()` reverses the most recent atomic batch (all events in the batch revert together)
• `redo()` re-applies a reversed batch
• `canUndo()` / `canRedo()` reflect the current stack state reactively
• A dependency cascade (move + N propagated dependents) is treated as one batch — one `undo()` call reverses it entirely
• Removing the `UndoRedoModule` causes undo/redo code to tree-shake out of the bundle

Blocked by

#28