Shortcut labels were rebuilt from scratch in the render body of every
component that shows one, which kept parseKeybinding running ~120x/sec
in a fully idle app.
- Cache the label layer per overrides object (WeakMap), so a keybinding
edit hands out a new object and therefore a fresh cache.
- Memoize parseKeybinding behind a bounded cache; binding strings come
from a fixed definition set plus user overrides.
- Hoist the per-call token/label object literals in normalizeKeyToken
and formatKeyToken to module constants.
