feat: add Undo and UndoOrExit action variants

[clansty]

Summary

• Add undo and undo-or-exit as new action variants for action chains
• undo: plain action, no-op if nothing to undo, always continues the chain
• undo-or-exit: compound action — undo one step if possible, exit if nothing to undo
• Route Enter/Escape/right-click through new process_actions method for consistent Undo support
• No implicit control flow — compound behavior is explicit in the action name

Closes #485
Related discussion: #481

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