feat: add URL deep-linking for DataGrids

- Add PageUrlState utility for URL ↔ state serialization
- Add column `id` field for unique column identification in URLs
- Update idSelector to return String for URL compatibility
- All DataGrid pages now support URL params: search, sort, order, id
- Browser URL updates via replaceState (no GoRouter rebuilds)
- Add FilterPanelSemantics for filter panel semantic IDs
- Add TESTING.md documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/features/control_room/containers/presentation/pages/containers_list_page.dart

@@ -1,9 +1,13 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart' hide Container;
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
 import 'package:tatlock_ui/core/api/api_client.dart';
 import 'package:tatlock_ui/features/control_room/containers/presentation/providers/containers_provider.dart';
 import 'package:tatlock_ui/features/control_room/containers/presentation/widgets/container_logs_viewer.dart';
 import 'package:tatlock_ui/features/control_room/containers/presentation/widgets/container_status_badge.dart';
+import 'package:tatlock_ui/routing/url_state.dart';
 import 'package:tatlock_ui/shared/components/data_grid/adapters/core_api_source.dart';
 import 'package:tatlock_ui/shared/components/data_grid/data_grid_exports.dart';
 
@@ -101,7 +105,10 @@ class ContainerPort {
 
 /// Page displaying the list of containers using DataGrid.
 class ContainersListPage extends ConsumerStatefulWidget {
-  const ContainersListPage({super.key});
+  const ContainersListPage({super.key, this.routerState});
+
+  /// Router state for URL deep-linking.
+  final GoRouterState? routerState;
 
   @override
   ConsumerState<ContainersListPage> createState() => _ContainersListPageState();
@@ -110,10 +117,18 @@ class ContainersListPage extends ConsumerStatefulWidget {
 class _ContainersListPageState extends ConsumerState<ContainersListPage> {
   late final StateNotifierProvider<DataGridController<ContainerData>,
       DataGridState<ContainerData>> _gridProvider;
+  late PageUrlState _urlState;
+  Timer? _urlSyncTimer;
 
   @override
   void initState() {
     super.initState();
+
+    // Parse URL state
+    _urlState = PageUrlState.fromQueryParams(
+      widget.routerState?.uri.queryParameters ?? {},
+    );
+
     final dio = ref.read(coreApiClientProvider);
     final source = CoreApiDataSource<ContainerData>(
       dio: dio,
@@ -121,17 +136,68 @@ class _ContainersListPageState extends ConsumerState<ContainersListPage> {
       fromJson: ContainerData.fromJson,
     );
 
+    // Initialize grid with URL state
     _gridProvider = dataGridProvider<ContainerData>(
       source: source,
       config: _buildConfig(),
       idSelector: (c) => c.id,
+      initialSearch: _urlState.search,
+      initialSortColumnIndex: _columnIndexForId(_urlState.sortColumn),
+      initialSortDescending: _urlState.sortDescending,
     );
   }
 
+  @override
+  void dispose() {
+    _urlSyncTimer?.cancel();
+    super.dispose();
+  }
+
+  /// Find column index by column ID.
+  int? _columnIndexForId(String? columnId) {
+    if (columnId == null) return null;
+    final columns = _buildConfig().columns;
+    for (var i = 0; i < columns.length; i++) {
+      if (columns[i].id == columnId) return i;
+    }
+    return null;
+  }
+
+  /// Get column ID by index.
+  String? _columnIdForIndex(int index) {
+    final columns = _buildConfig().columns;
+    if (index >= 0 && index < columns.length) {
+      return columns[index].id;
+    }
+    return null;
+  }
+
+  /// Schedule URL sync with debounce.
+  void _scheduleUrlSync() {
+    _urlSyncTimer?.cancel();
+    _urlSyncTimer = Timer(const Duration(milliseconds: 500), _syncUrlParams);
+  }
+
+  /// Sync current state to URL.
+  void _syncUrlParams() {
+    final state = ref.read(_gridProvider);
+
+    final params = PageUrlState(
+      search: state.searchQuery.isEmpty ? null : state.searchQuery,
+      sortColumn: state.sortColumnIndex != null
+          ? _columnIdForIndex(state.sortColumnIndex!)
+          : null,
+      sortDescending: state.sortDescending,
+    ).toQueryParams();
+
+    updateBrowserUrlParams(params);
+  }
+
   DataGridConfig<ContainerData> _buildConfig() {
     return DataGridConfig<ContainerData>(
       columns: [
         DataGridColumn<ContainerData>(
+          id: 'container',
           header: 'Container',
           valueBuilder: (c) => '${c.name} ${c.image}',
           sortable: true,
@@ -140,12 +206,14 @@ class _ContainersListPageState extends ConsumerState<ContainersListPage> {
           cellBuilder: (context, c) => _ContainerCell(container: c),
         ),
         DataGridColumn<ContainerData>(
+          id: 'ports',
           header: 'Ports',
           valueBuilder: (c) => c.ports.map((p) => p.formatted).join(', '),
           width: const DataGridColumnWidth.flex(1),
           cellBuilder: (context, c) => _PortsCell(ports: c.ports),
         ),
         DataGridColumn<ContainerData>(
+          id: 'status',
           header: 'Status',
           valueBuilder: (c) => c.displayStatus,
           width: const DataGridColumnWidth.fixed(160),
@@ -211,6 +279,15 @@ class _ContainersListPageState extends ConsumerState<ContainersListPage> {
   Widget build(BuildContext context) {
     final colorScheme = Theme.of(context).colorScheme;
 
+    // Listen for grid state changes to sync URL
+    ref.listen(_gridProvider, (previous, next) {
+      if (previous?.searchQuery != next.searchQuery ||
+          previous?.sortColumnIndex != next.sortColumnIndex ||
+          previous?.sortDescending != next.sortDescending) {
+        _scheduleUrlSync();
+      }
+    });
+
     // Listen for container action results to show snackbars
     ref.listen<AsyncValue<void>>(containerActionsProvider, (previous, next) {
       if (previous?.isLoading == true && !next.isLoading) {