…ncestor isFocused, explicit keybinding helpers

- Rename src/core/focus/useFocus.ts → useFocusNode.ts; rename export to useFocusNode
- New useFocus() reads focused state from nearest FocusGroup ancestor without registering a node
- isFocused() now walks the ancestor chain instead of exact-matching, so container nodes report focused=true when any descendant has focus
- FocusGroup: remove direction/navigable props and auto-keybinding logic; keybindings prop now accepts Keybindings | ((helpers) => Keybindings); expose next/prev/escape helpers
- Export FocusGroupHelpers type from core/focus/index.ts and src/index.ts
- Export useFocusNode in public API

…mers

Update imports and call sites in FocusTrap, Select, Autocomplete, MultiSelect,
Confirm, TextInput, CommandPalette, Modal, and Viewport. No logic changes.

…sGroup to explicit keybindings

- Replace useFocus() with useFocusNode() in all example components
- Remove direction/navigable props from FocusGroup; add explicit keybinding functions
- controlled-focus: remove navigable={false}
- focus-trap, use-focus, navigable-menu: vertical j/k/down/up via helpers
- horizontal-tabs: horizontal h/l/left/right via helpers
- command-palette: tab/shift+tab via helpers

focus.mdx:
- Rewrite prose to explain FocusGroup as scoping primitive and the useFocus/useFocusNode split
- Update useFocus() API: no parameters, reads from nearest FocusGroup, focused=true when any descendant has focus
- Add useFocusNode() API reference
- Add FocusGroup keybindings function form and FocusGroupHelpers type table
- Remove direction/navigable from FocusGroup prop table
- Update all code examples to use explicit keybindings and correct hooks
- Add "Panel borders with useFocus" example

core-concepts.mdx:
- Update "How nodes register" section: useFocus → useFocusNode, add useFocus description
- Update FocusGroup section: remove auto-keybinding description, reflect explicit helpers

README: add useFocusNode to feature bullet

…rOpen theme token

- Replace horizontal tabs example with collapsible file tree in focus.mdx
- Fix navigateSibling to focus first/last child when no child of the group is
  currently focused, instead of returning early
- Add indicatorOpen to GigglesTheme (default '▼') alongside existing indicator ('▶')
- Update theme.mdx to document indicatorOpen
- Use indicator/indicatorOpen from theme in file tree example

unregisterNode's setFocusedId updater previously returned node.parentId
directly, which could be a deleted node if React ran parent cleanup effects
before child cleanup effects. Add a parentMapRef that retains parent
relationships for all registered nodes (never deleted), so the updater can
walk up the chain and find the first ancestor that still exists in the live
nodesRef — regardless of cleanup order.

… explicit keybindings