Extract compilable debugging examples from debugging_tracing.md (10/12)

Created debugging_patterns.dart with 8 regions:
- track_rebuild_frequency
- isolate_problem
- measure_rebuild_cost
- profile_memory
- find_excessive_watch_calls
- custom_watch_wrapper_logging
- conditional_tracing
- detect_watch_ordering_violations

Extracted only GOOD (compilable) debugging examples.
BAD examples showing errors are intentionally left inline in markdown.
Examples compile successfully (10 info warnings only).

Progress: 10 files done, 2 remaining.

Author: escamoteur

code_samples/lib/watch_it/debugging_patterns.dart

@@ -0,0 +1,182 @@
+// ignore_for_file: unused_local_variable, unreachable_from_main, undefined_class, unused_element, dead_code
+import 'package:flutter/material.dart';
+import 'package:watch_it/watch_it.dart';
+import '_shared/stubs.dart';
+
+final di = GetIt.instance;
+
+// NOTE: This file contains GOOD debugging examples only.
+// BAD examples that demonstrate errors are left inline in the documentation.
+
+// #region track_rebuild_frequency
+class TrackRebuildFrequency extends WatchingWidget {
+  static int _buildCount = 0;
+
+  @override
+  Widget build(BuildContext context) {
+    print('TodoList rebuild #${++_buildCount} at ${DateTime.now()}');
+
+    final todos = watchValue((TodoManager m) => m.todos);
+    return ListView.builder(
+      itemCount: todos.length,
+      itemBuilder: (context, index) => Text(todos[index].title),
+    );
+  }
+}
+// #endregion track_rebuild_frequency
+
+// #region isolate_problem
+// Minimal test widget
+class IsolateProblemDebugWidget extends WatchingWidget {
+  @override
+  Widget build(BuildContext context) {
+    print('DebugWidget.build()');
+
+    final data = watchValue((DataManager m) => m.data);
+    print('Data value: $data');
+
+    return Text('Data: $data');
+  }
+}
+// #endregion isolate_problem
+
+// #region measure_rebuild_cost
+class MeasureRebuildCost extends WatchingWidget {
+  @override
+  Widget build(BuildContext context) {
+    final stopwatch = Stopwatch()..start();
+
+    final data = watchValue((DataManager m) => m.data);
+    // ... build UI
+
+    stopwatch.stop();
+    print('ExpensiveWidget.build() took ${stopwatch.elapsedMicroseconds}μs');
+
+    return Text(data);
+  }
+}
+// #endregion measure_rebuild_cost
+
+// #region profile_memory
+class MemoryProfiler {
+  static void logCurrentUsage(String label) {
+    // In real app, use dart:developer
+    print('[$label] Memory check');
+  }
+}
+
+class ProfileMemoryWidget extends WatchingWidget {
+  @override
+  Widget build(BuildContext context) {
+    MemoryProfiler.logCurrentUsage('TodoList.build');
+
+    final todos = watchValue((TodoManager m) => m.todos);
+    return ListView.builder(
+      itemCount: todos.length,
+      itemBuilder: (context, index) => Text(todos[index].title),
+    );
+  }
+}
+// #endregion profile_memory
+
+// #region find_excessive_watch_calls
+class FindExcessiveWatchCalls extends WatchingWidget {
+  static final _watchCalls = <String, int>{};
+
+  T _profiledWatch<T>(String label, T Function() fn) {
+    _watchCalls[label] = (_watchCalls[label] ?? 0) + 1;
+    if (_watchCalls[label]! % 100 == 0) {
+      print('$label called ${_watchCalls[label]} times');
+    }
+    return fn();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final todos = _profiledWatch(
+      'todos',
+      () => watchValue((TodoManager m) => m.todos),
+    );
+
+    return ListView.builder(
+      itemCount: todos.length,
+      itemBuilder: (context, index) => Text(todos[index].title),
+    );
+  }
+}
+// #endregion find_excessive_watch_calls
+
+// #region custom_watch_wrapper_logging
+extension WatchDebug on WatchingWidget {
+  T debugWatch<T>(
+    String label,
+    T Function() watchFn,
+  ) {
+    print('[WATCH] $label - subscribing');
+    final result = watchFn();
+    print('[WATCH] $label - value: $result');
+    return result;
+  }
+}
+
+class CustomWatchWrapperWidget extends WatchingWidget {
+  @override
+  Widget build(BuildContext context) {
+    final todos = debugWatch(
+      'todos',
+      () => watchValue((TodoManager m) => m.todos),
+    );
+
+    return ListView.builder(
+      itemCount: todos.length,
+      itemBuilder: (context, index) => Text(todos[index].title),
+    );
+  }
+}
+// #endregion custom_watch_wrapper_logging
+
+// #region conditional_tracing
+class ConditionalTracingWidget extends WatchingWidget {
+  final bool enableTracing;
+
+  ConditionalTracingWidget({this.enableTracing = false});
+
+  @override
+  Widget build(BuildContext context) {
+    if (enableTracing) {
+      print('Building $runtimeType');
+    }
+
+    final data = watchValue((DataManager m) => m.data);
+
+    if (enableTracing) {
+      print('Data value: $data');
+    }
+
+    return Text('$data');
+  }
+}
+// #endregion conditional_tracing
+
+// #region detect_watch_ordering_violations
+class StrictWatchingWidget extends WatchingWidget {
+  final _watchLabels = <String>[];
+
+  T _strictWatch<T>(String label, T Function() fn) {
+    if (_watchLabels.contains(label)) {
+      throw StateError('Duplicate watch call: $label');
+    }
+    _watchLabels.add(label);
+    return fn();
+  }
+
+  // Use in subclasses:
+  // final todos = _strictWatch('todos', () => watchValue(...));
+  @override
+  Widget build(BuildContext context) {
+    return Container();
+  }
+}
+// #endregion detect_watch_ordering_violations
+
+void main() {}