- Fix Multi+Directories mode: Enter now confirms toggled selections
  instead of always navigating into the highlighted directory
- Clear _selectedPaths on directory navigation to prevent stale
  cross-directory paths from being emitted on confirmation
- Auto-include highlighted file in multi-select Enter confirmation
- Fix breadcrumb truncation crash (ArgumentOutOfRangeException) when
  render width is 4 or less
- Fix '/' then Backspace leaving user stuck in filter mode by moving
  empty-text check outside the handled guard
- Allow space characters in filter text by removing Spacebar
  interception in filter mode
- Fix entry truncation crash when contentWidth is 0
- Clamp availableForList to prevent negative listHeight in render
- Use SetFocus instead of PushFocus in gallery page to prevent
  unbounded focus stack growth
- Replace _entries.Count==0 sentinel with _isLoaded flag so empty
  directories don't re-trigger LoadDirectory on every focus
- Catch IOException alongside UnauthorizedAccessException in
  DefaultFileSystemProvider for network/device errors
- Add !_disposed guards on _selectionConfirmed and _cancelled
  Subject emissions to match Invalidate/EmitDirectoryChanged pattern
- Deactivate filter input before completing subjects in Dispose
- Align x-offset: breadcrumb, filter bar, and footer now render at
  x=0 consistently with file list entries
- Fix Slash_ActivatesFilterMode test to verify filter mode was
  actually entered before asserting cancellation
- Add tests for backspace-exits-empty-filter and Multi+Directories
  toggle-then-confirm workflows

- Add docs/components/file-picker-node.md covering usage, picker and
  selection modes, keyboard shortcuts, filtering, glob filters,
  styling, IFileSystemProvider testing, and full API reference
- Register FilePickerNode in the VitePress sidebar and components index
- Add gallery-file-picker.tape VHS script and recorded GIF showing
  directory navigation, type-to-filter, file and folder selection
- Lazy-load the directory on first render so unfocused pickers show
  content instead of "Empty directory" before receiving focus

- Add Layouts.FilePicker(startPath) factory matching the established
  pattern for SelectionList, Modal, Wizard, and other interactive nodes
- Switch docs examples and gallery demo page to use the factory
- Add unit test covering both the explicit-path and default overloads

Load lifecycle:
- Latch _isLoaded on failed loads so a nonexistent start path no longer
  retries filesystem I/O on every render frame; WithStartPath resets the
  latch to allow reloading
- Collapse three lazy-load sites into EnsureLoaded(); initial lazy loads
  no longer emit DirectoryChanged (navigation-only signal)

Filter input:
- Only ApplyFilter when the filter text actually changes — cursor
  movement (Left/Right) no longer resets the list highlight and scroll
- Unhandled keys (Tab, F-keys, Ctrl chords) now bubble instead of being
  swallowed and silently closing an empty filter; Backspace on an empty
  filter still exits filter mode explicitly
- Alt/Ctrl chords no longer hijacked into filter activation
- Filter input now receives OnFocused/OnBlurred so the cursor blink
  timer starts and stops correctly; its Invalidated is bridged to the
  picker's so blink frames repaint; TimeProvider is threaded through
  for testability
- LoadDirectory exits filter mode with full deactivation instead of
  just flipping the flag (no more orphaned blink timer)

Multi-select model:
- Selections persist across directory navigation; footer shows a
  running count (e.g. "3 selected")
- Enter on an untoggled directory always navigates (drill-in restored);
  Enter on a toggled directory confirms — behavior is predictable from
  the visible [x] on the highlighted row
- Confirming no longer mutates the toggle set; the highlighted file is
  appended to the emitted list only

Scroll math:
- EnsureVisible now tracks the rows actually rendered (_effectiveRows),
  fixing blind navigation when the parent allocates less space than the
  configured visible rows
- _scrollOffset re-clamped in Render so enlarging the terminal no
  longer strands the view past the end of the list
- Measure reserves a row for the empty-state message so it no longer
  overwrites the footer

Misc:
- DefaultFileSystemProvider.GetParentDirectory trims trailing
  separators and guards ArgumentException/IOException
- EmitSelectionConfirmed helper for consistent disposed-guarding
- Filter bar width off-by-one and empty-state x-offset cleanup
- Stale Cancelled XML doc corrected
- Gallery menu hint updated to [1-6] Quick Select; GIF re-recorded
- Docs updated for the new multi-select semantics; doc fake provider
  example now sorts per the GetEntries contract
- 9 new tests: cursor-move highlight preservation, unhandled-key
  bubbling, space-in-filter, cross-directory selection persistence,
  confirm non-mutation, All+Multi mode, untoggled-dir navigation,
  WithStartPath reload, bad-path no-retry; hidden-file tests made
  discriminating

The OnActivate/OnDeactivate tree-walk pattern-matched on the concrete
LayoutNode class at every dispatch site, so components implementing
ILayoutNode interfaces directly (FilePickerNode, SelectionListNode)
never received lifecycle calls. Their OnActivate/OnDeactivate methods
were dead code, which is why FilePickerNode had to lazy-load in
Render() as a workaround.

- LayoutNode now implements the existing IActivatableNode interface
  (its virtual OnActivate/OnDeactivate already satisfy it)
- All 36 activation dispatch sites across ContainerNode, GridNode,
  PanelNode, ModalNode, ConditionalNode, DynamicLayoutNode,
  KeyedDynamicLayoutNode, ReactiveLayoutNode, and ReactivePage now
  pattern-match on IActivatableNode instead of LayoutNode — behavior
  for LayoutNode subclasses is unchanged, and bare-interface nodes
  now participate in the page lifecycle
- FilePickerNode and SelectionListNode implement IActivatableNode,
  so directory loading happens at page activation instead of first
  render, and embedded input timers pause on navigation away
- Traversal-only LayoutNode checks (FocusManager child collection,
  WizardNode tree walks) are intentionally unchanged
- Render-time EnsureLoaded() retained as a fallback for nodes
  attached outside the activation flow

Adds tests verifying activation propagates through VerticalLayout and
GridNode to an interface-based FilePickerNode child.

"[BS] Up" was ambiguous — use "[Backspace] Up" in all four footer hint
variants. Re-record the gallery GIF to match.

- Custom-components guide now explains that lifecycle dispatch goes
  through IActivatableNode (LayoutNode implements it; interface-based
  components implement it directly) and the container propagation
  example matches on the interface instead of the concrete class —
  the old sample taught the exact gap fixed in d390749
- Replace the Timer/.Elapsed event example with Observable.Interval +
  TimeProvider, and the System.Reactive types (IObservable,
  BooleanDisposable) with R3 equivalents, per project conventions
- Add missing TextAreaNode and CopyableTextNode entries to the docs
  sidebar Input Components section