fix: keep the deck out of focus traversal

[spydon]

Description

On the web, starting any deck throws RenderBox was not laid out once at startup:

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞══════════════════════════════════════
The following assertion was thrown while dispatching notifications for
WidgetsBindingObserver.didChangeViewFocus:
Assertion failed: hasSize
"RenderBox was not laid out: RenderSemanticsAnnotations#53775 NEEDS-LAYOUT
NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE"

FlutterDeckControlsListener wraps the deck in Focus(autofocus: true). The autofocus is applied from a microtask before the first layout, and on the web that synchronously round trips through the engine:

FocusManager.applyFocusChangesIfNeeded
  -> View._scopeFocusChangeListener
  -> PlatformDispatcher.requestViewFocusChange
  -> DomElement.focusWithoutScroll
  -> WidgetsBinding.handleViewFocusChanged
  -> View.didChangeViewFocus
  -> FocusTraversalPolicy.findFirstFocus
  -> _sortAllDescendants

_sortAllDescendants reads FocusNode.rect, which reads RenderBox.size and asserts hasSize. No layout has run at that point, so the assertion fires.

Fix

The node exists so the deck receives key events, not so it can be a tab stop, so it is marked skipTraversal. It is then left out of the traversal sort and never has its rect read, while it still holds focus and still receives shortcuts.

Testing

• Added should focus the deck without joining focus traversal, which fails without the fix and passes with it.
• The full flutter_deck suite passes (209 tests). Removing the focus entirely breaks 3 of the existing keyboard tests, so the deck genuinely still receives keyboard input.
• Verified in a real deck on Flutter 3.47.0 stable with flutter run -d chrome: 1 exception at startup before the change, 0 after.
• Also verified with a focusable widget (an ElevatedButton) on the first slide, to check the fix does not depend on the deck node being the only focus node: 1 exception before the change, 0 after.

[mkobuolys]

@spydon ❤️