fix(webf): guard semantics proxies and widget layout paths

[andycall]

Summary

• skip dirty transparent proxy wrappers during semantics traversal using a runtime layout-dirty signal instead of debug-only checks
• restore portal-mounted WidgetElement width fallback for detached render subtrees
• allow block/flex atomic placeholders in inline-item collection and add focused regression coverage

Testing

• /Users/andycall/workspace/flutter_341/bin/flutter --no-color test /Users/andycall/workspace/webf3/webf/test/src/html/input_checked_test.dart
• /Users/andycall/workspace/flutter_341/bin/flutter --no-color test /Users/andycall/workspace/webf3/webf/test/src/html/accessibility_test.dart /Users/andycall/workspace/webf3/webf/test/src/html/accessibility_semantics_dirty_wrapper_test.dart
• /Users/andycall/workspace/flutter_341/bin/flutter --no-color test /Users/andycall/workspace/webf3/webf/test/src/rendering/render_widget_portal_constraints_test.dart /Users/andycall/workspace/webf3/webf/test/src/rendering/inline_atomic_block_assertion_test.dart /Users/andycall/workspace/webf3/webf/test/src/rendering/inline_span_local_ifc_under_widget_test.dart
• cd /Users/andycall/workspace/webf3/integration_tests && npm run integration -- specs/rendering/widget_cupertino_context_menu_popup_preview_width.ts specs/rendering/widget_portal_cupertino_modal_popup_width_not_clamped.ts

[vercel]

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Project	Deployment	Actions	Updated (UTC)
use-case	Ready	Preview, Comment	Apr 3, 2026 7:47am
vue_usecase	Ready	Preview, Comment	Apr 3, 2026 7:47am