Implement BufdirReportSectionWidget collapsible card

Prefer a custom implementation over raw ExpansionTile to have full control over semantics and animation. Use AnimationController with CurvedAnimation(curve: Curves.easeInOut, duration: Duration(milliseconds: 250)). The header Row should contain: [Expanded(child: semanticTitle), if (hasIssues) ValidationBadgeWidget(...), AnimatedRotation(chevron)]. For the semantic heading: wrap the title Text in Semantics(header: true, label: 'Section: ${data.title}').

The heading level (h2/h3) cannot be directly expressed in Flutter semantics as an ARIA heading level — use the label prefix 'Section:' as the accessible convention for this app. For the validation badge, create a small private _ValidationBadge StatelessWidget with two colored pill containers side by side for errors and warnings — keep it reusable for future use in BufdirPreviewScreen. Use shrinkWrap: true and NeverScrollableScrollPhysics on the inner ListView to prevent nested scroll conflicts. Test in a ConstrainedBox(maxHeight: 600) to simulate being inside a parent scroll view.

Write widget tests in test/features/bufdir/widgets/bufdir_report_section_widget_test.dart. Cover: (1) in collapsed state, BufdirFieldRowWidget children are not present in widget tree (find.byType(BufdirFieldRowWidget) returns zero); (2) after tap on header, children appear (findsNWidgets(data.fields.length)); (3) second tap collapses again; (4) validation badge is absent when errorCount=0 and warningCount=0; (5) validation badge shows correct counts from BufdirValidationSummary; (6) Semantics header flag is set on title widget (use tester.getSemantics and check SemanticsFlag.isHeader); (7) no RenderFlex overflow with 20 field rows. Golden tests for all three specified states. Run flutter test --update-goldens to capture baselines.