Implement AccessibleSearchInputField widget

Wrap the root of the widget with FocusTraversalGroup using ReadingOrderTraversalPolicy to enforce field → clear button focus order. Use Semantics with label and hint parameters directly — avoid MergeSemantics unless you need to collapse child nodes intentionally. For the clear button: use AnimatedOpacity or Visibility(maintainState: false) conditioned on controller.text.isNotEmpty so it disappears from both the visual layer and the semantics tree when not relevant. Do not use textField: true on the Semantics node wrapping AppTextField — AppTextField already provides its own semantics from the underlying TextField; adding textField: true on a parent node would create a duplicate announcement.

Instead, annotate only the outer container with label and hint, and rely on the inner TextField's native semantics for text-field role announcement. Expose a FocusNode parameter so the parent screen can programmatically focus the field (e.g., on screen load).

Widget-test AccessibleSearchInputField: (1) renders with AppTextField as a descendant, (2) Semantics node has correct label and hint via tester.getSemantics(), (3) clear button Semantics node has label 'Clear search', (4) clear button is absent from semantics tree when field is empty, (5) onChanged fires when text is entered, (6) onClear fires when clear button is tapped and field is empty afterward, (7) FocusTraversalGroup orders field before clear button. Use flutter_test SemanticsHandle for semantic assertions. No golden tests required for this task.