Add undo support

[cobalt-arc-dev]

Fixes #23.

Summary:

• Add an undo snapshot stack to the editor and wire Ctrl+Z / Cmd+Z to restore the previous automaton state.
• Record undo snapshots before user-visible mutations: adding/deleting states, renaming states/transitions, changing accepting/start states, creating/retargeting transitions, and committed state drags.
• Keep selection-only actions out of the undo history.

Validation:

• node --check site/js/editor.js
• find site/js -name '*.js' -print0 | xargs -0 -n1 node --check
• git diff --check
• Node VM harness covering undo of state creation and undo of state position restoration.