perf(code-block): memoize Prism tokenization and token color merge

Both Prism.tokenize() and the defaultTokenColors spread ran
unconditionally on every render. Wrap colors in useMemo keyed on
tokenColors, and content in useMemo keyed on children/grammar/colors,
so re-tokenization is skipped when the code and language haven't changed.

refactor(ui): use TextInput in CommandPalette instead of reimplementi…

…ng input

Replace manual character/backspace handling and cursor rendering with
TextInput. Navigation keys (up/down/enter/escape) bubble from TextInput
to a parent FocusScope. Fixes cursor appearing at end of placeholder.

fix(focus): eliminate focus flash on mount by registering nodes durin…

…g render

Move node registration from useEffect to the render phase so that
useSyncExternalStore's getSnapshot returns the correct hasFocus value
on the first render. This eliminates the visible one-frame lag when
opening collapsible sections or returning to router screens.

- Register nodes silently during render (no subscriber notifications)
- Flush deferred notifications in useLayoutEffect before paint
- Extract passive-clearing into a shared helper for both silent and
  normal focus transitions
- Track focusKey on FocusNode to handle key changes without destroying
  the node's children
- Re-register in useLayoutEffect setup for React Strict Mode compat

docs(input): improve description and section introductions

docs: update markdown context file for agents

fix(command-palette): make useKeybindingRegistry reactive

Add version counter to FocusStore and subscribe via useSyncExternalStore
so HintsBar updates on focus/node changes. Deduplicate available bindings
by key to prevent duplicates from ancestor scopes.

docs(command-palette): add action feedback to example, note default U…

…I has no hints

chore: bump version to 0.5.3

fix(ui): backspace key not registering

docs: fix stale useFocus import and missing Terminal import in router