fix(hot-reload): drive Flutter Tools' reloadSources/hotRestart VM services so hot_reload/hot_restart actually apply changes

[pierre-cheneau]

Problem

hot_reload / hot_restart report success but never apply source changes.

FlutterSkillClient.hotReload() calls the VM's raw reloadSources(isolateId).
A Flutter app embeds no kernel compiler, so the VM answers
ReloadReport{success:false, notices:["Error while starting Kernel isolate task"]};
the report is ignored and the tool returns "Hot reload triggered" while nothing
recompiled. hotRestart() is the same call (or throws UnsupportedError
claiming the VM Service cannot restart — it can, see below).

Fix

The flutter run process that launched the app owns the incremental compiler and
registers VM services reloadSources and hotRestart (alias "Flutter Tools",
callable as sN.reloadSources / sN.hotRestart, advertised on the Service
stream — the VM replays existing registrations on streamListen). That is exactly
what DevTools and the IDE plugins call.

• FlutterSkillClient tracks those registrations (connect + reconnect) and
  hotReload/hotRestart invoke them → real recompile + reload + reassemble.
• Without an attached tool (bare VM URI of a profile/release/detached app),
  hotReload falls back to the raw RPC and checks the report, surfacing a
  clear "launch the app with flutter run" error; hotRestart explains it needs
  the tool.
• After a restart the main isolate is recreated: re-bind to the new isolate
  (id must differ — the old one may still be listed), then wait for
  ext.flutter.didSendFirstFrameEvent so the widget tree is populated for the
  caller's next call. (didSendFirstFrameRasterizedEvent never flips on Windows
  desktop; the value is a JSON bool.) _call also self-heals on a stale isolate
  (Sentinel: Collected), so a restart done from the terminal no longer breaks
  the session.
• On Windows, hold until the wall-clock second rolls over before returning: Dart
  truncates file mtimes to whole seconds there, so flutter_tools'
  ProjectFileInvalidator ignores edits made in the same second as its last
  compile ("Compiling dart to kernel with 0 updated files"). Agents edit and
  reload back-to-back, so this hit every second call. (Reported upstream to
  Flutter separately.)
• Responses now say what happened: Hot reload performed by Flutter Tools (330ms).

Verification

Minimal app showing a MARKER_Vn constant, launched with flutter run -d windows
(Flutter 3.44.9, Windows 10). Script drives the server over MCP stdio:
connect → edit → hot_reload → get_text_content → edit → hot_restart →
get_text_content.

Server	Result
0.9.34 release / HEAD	0/10 — text never changes, flutter run logs nothing
this PR	10/10 — Reloaded 1 of 756 libraries / Restarted application, every compile "1 updated files"

dart analyze clean on the touched files; no existing tests reference the old behaviour.

[pierre-cheneau]

The Windows mtime-granularity behaviour that motivates the _settleCompileClock wait is now reported upstream: flutter/flutter#191198 ("Hot reload/restart on Windows ignores files edited in the same second as the previous compile"). If Flutter fixes it in the tool, the guard here becomes a harmless ≤1 s wait that can be dropped for SDKs that carry the fix.

[pierre-cheneau]

Follow-up: #59 (synthetic input engine — single tap activation, timestamped pointer events, drag hold, hittable flag, screenshot geometry) is stacked on this branch; it will be rebased onto main once this lands.