fix: use common Flutter render pipeline on Linux

[local-sandbox-agent]

Problem

Linux takes its frame timing from Flutter's SchedulerBinding, but the Flutter-synchronized path also bypassed the normal Dart render callback and rendered directly in native code. That meant FilamentApp.render() did not run, so Dart request-frame hooks such as camera controllers and procedural updates were skipped.

The separate native render path was not required by Flutter or Filament; only the Linux frame source and external-texture notification need to be platform-specific.

Structure

FrameScheduler continues to own frame timing for every native platform:

• direct native timing ticks
• debug receive-port ticks
• Flutter-synchronized Linux ticks

All three modes now dispatch the same Dart frame handler. On Linux, Flutter's persistent frame callback applies the target-FPS deadline and the common in-flight guard, then dispatches the same _renderFrame() handler as the native display-link modes.

That common callback:

1. runs Dart request-frame hooks through FilamentApp.render()
2. queues and awaits rendering on Thermion's render thread
3. publishes the completed frame to Flutter

Linux keeps one presentation-specific optimization: after the common render future completes, a direct FFI call marks all registered Linux external textures available. Texture notification remains separate from scheduling and rendering.

The hooks-only callback and the separate native Linux render path have been removed. Non-Linux scheduling behavior is unchanged.

The native FrameScheduler API is now timing-only. Its C++ and C interfaces use tick terminology, and the C entry point is explicitly named FrameScheduler_startWithCallback. The unused rendering API and its global render state were deleted:

• FrameScheduler_setRenderThread
• FrameScheduler_setRenderManager
• FrameScheduler_setPostRenderCallback
• FrameScheduler_requestRender
• FrameScheduler_startNativeRenderLoop

Dart now distinguishes incoming platform ticks from dispatched frame handlers (setFrameHandler, _tryDispatchFrame, and dispatchedFrameCount).

Validation

• flutter analyze in thermion_flutter: no issues
• full thermion_flutter test suite: 29 tests passed
• focused Flutter-synchronized scheduler test covers target-FPS gating, slow-render backpressure, pause/resume, stop, and persistent-callback reuse across restart
• quickstart analysis and web build: passed
• native code compiled through the Flutter test harness
• git diff --check: passed

[nmfisher]

Closing this sandbox-app-authored PR so it can be replaced by an equivalent PR authored directly by @nmfisher. The branch and commits are unchanged.

[nmfisher]

Superseded by #278, opened directly by @nmfisher from the same branch and commits with an updated description.