Add support for modern inspector

[vmoroz]

Description

This PR adds support for the React Native DevTools to the React Native for Windows. The RN DevTools are also known as "modern inspector" or "fusebox", and based on the Chrome DevTools. The implementation follows the React Native modern inspector requirements implemented for Android and iOS platforms.

The PR combines work from several developers:

• @rozele created the original draft PR: Wire up Fusebox in react-native-windows #13008;
• @iamAbhi-916 made a number of important improvements on top of the early work based on Wire up Fusebox in react-native-windows #13008 in draft PR: Draft Debugger #15139
• @acoates-ms implemented the Composition-based Debugger UI.
• @iamAbhi-916 did a lot of work to ensure that the modern inspector works end-to-end for the "Composition" and "Paper' UIs, discovering and fixing many issues.

Type of Change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Why

We want RNW developers to enjoy the same modern developer experience as Android and iOS React Native developers.

What

• Runtime plumbing

  • Removed some our temporary ReactCommon overrides now that upstream behavior works on Windows.
  • Extended ReactOptions, ReactHost, ReactInstanceWin, and OInstance so they surface the modern inspector target and propagate pause/resume notifications to app code.
  • ReactHost only registers itself for debugging by adding the inspector "page" when the ReactSettings have the UseDirectDebugger set to true.
  • Use dedicated ReactInspectorThread thread for the inspector communication synchronization. (Android and iOS use UI thread)
  • Introduced DebuggerNotifications.h to send notifications about the debugger state changes to allow UI updates.
  • Added ReactInspectorPackagerConnectionDelegate to let Metro drive the modern inspector handshake for RNW.

• Hermes integration

  • Added HermesRuntimeTargetDelegate and HermesRuntimeAgentDelegate, mirroring upstream React Native code for Hermes but targeting the hermes-windows C API.
  • Updated RuntimeHolder and HermesRuntimeHolder to create/use inspector delegates only when needed and to tear them down safely.
  • Fixed bad script name in JsiApi.cpp that caused DevTools from loading scripts.

• UI / UX

  • Composition: introduced DebuggerUIIsland so paused apps show overlays and selection handles.
  • Paper: mirrored the same overlay/selection experience inside ReactRootView.
  • Resolved the DebuggingOverlayComponentView recursion that caused stack overflows when selecting deep component trees.
  • Hardened TaskDispatchThread shutdown so we don’t dereference freed objects when the JS thread disposes the dispatcher.

• Dev tooling

  • DevSupportManager now manages modern inspector connections alongside legacy tooling.
  • Metro’s J shortcut opens the modern inspector for RNW targets just like other platforms.
  • NetworkIOAgent support is still pending: implement loadNetworkResource on ReactInspectorHostTargetDelegate (ReactHost.cpp) to proxy requests via a Windows HTTP helper (e.g. reuse GetJavaScriptFromServerAsync) and stream results back through the provided NetworkRequestListener.

• Documentation

  • Added modern-inspector-details.md capturing the architecture, usage, and follow-up work for the new inspector integration.

Testing

• Playground (Paper & Composition): yarn start, press J in Metro to attach the inspector, verified source debugging, console evaluation, CPU and memory profiling, and component tree selection with Hermes.
• Chakra runtimes remain on legacy debugging flows.

Changelog

• yes - include in release notes.

Add Modern Inspector support (JS debugger, console, CPU/Memory profilers, component tree) for React Native for Windows applications.

Microsoft Reviewers: Open in CodeFlow