DevTools "Count widget builds" only works in debug mode, not in --profile mode

### Steps to reproduce

1. flutter run --profile --track-widget-creation
2. Open DevTools → Performance
3. The "Count widget builds" checkbox is greyed out.

### Expected results

The flag name `--track-widget-creation` suggests this should still be
available in profile mode. Documentation should clarify that widget build tracking is available debug-only.

### Actual results

I could not check the box to display the widget rebuilds, and the tooltip was not any help either. Documentation never mentions this. AI doesn't know.

### Code sample

Code sample

```dart
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### Screenshots or Video

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### Logs

Logs

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### Flutter Doctor output

Doctor output

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```

从 --track-widget-creation 的文档以及 DevTools Performance 页面中的“Count widget builds”控件开始，使用 `flutter run --profile --track-widget-creation` 重现复选框被禁用的情况。完成标准是文档和工具提示明确说明 widget 构建跟踪仅在 debug 模式下可用，包括 profile 模式下的行为。

由索引模型根据 Issue 内容生成。