feat: add Ask UI runtime widget bounds

[drown0315]

Summary

Adds an ask_ui_runtime Flutter package and wires Bridge/Web marker placement to real app-side widget bounds. This lets Selection Comment markers render inside the selected widget area instead of relying on unavailable Flutter Inspector JSON geometry.

What Changed

• Added packages/ask_ui_runtime with registerAskUiRuntime() to register debug-only Ask UI VM Service extensions.
• Added ext.ask_ui.widgetBounds support in Bridge and converts Flutter logical bounds to device-screen coordinates with devicePixelRatio.
• Updated Widget Tree serialization to carry bounds from the runtime extension.
• Moved Selection Comment overlay marker placement into the device frame and positions markers inside widget bounds.
• Documented runtime package setup and the widget bounds extension contract.

Why

Flutter Inspector tree JSON does not expose global widget origins. The runtime extension runs inside the target Flutter app and can use RenderBox.localToGlobal(Offset.zero) to return reliable bounds for Inspector widget ids.

Verification

• flutter analyze in packages/ask_ui_runtime
• flutter test in packages/ask_ui_runtime
• dart analyze in apps/bridge
• dart test in apps/bridge
• npm test -- --run in apps/web
• npm run build in apps/web