Spike: prototype MediaPipe Face Landmarker as ARCore replacement for head tracking

Why

#629's research into head-tracking tuning surfaced a bigger question than smoothing math: the two real shipped apps solving this exact problem for this population (Eva Facial Mouse, Google Project Gameface) don't use iOS's PID-controller approach at all, and more importantly, Project Gameface — Google's own current reference implementation for accessibility cursor control — doesn't use ARCore Augmented Faces, the API Vocable Android is built on. It uses MediaPipe Face Landmarker instead: richer data (468 landmarks + 52 blendshapes vs. ARCore's single nose-tip pose), no ARCore device-support gate, free/open-source, actively maintained by Google for this exact use case. Before investing further in tuning the current ARCore-based smoothing, we should validate whether swapping the tracking backend is viable and worth the larger migration.

Scope

• Build a MediaPipe-based tracking path behind a debug flag, side-by-side with the current ARCore path — this validates feasibility and feel, not a hardened production swap.
• CameraX + MediaPipe FaceLandmarker feeding the existing FaceTrackingViewModel-shaped vector output, unchanged downstream (convertCoordSystems, GazePointer, GazeInteractionManager, dwell-click all stay as-is).
• Side-by-side toggle so the MediaPipe path and current ARCore path can be compared on the same device.
• If it proves out, this branch becomes the basis for the production migration (tracked as a separate follow-up issue), not a throwaway to be discarded.

Acceptance Criteria

• Prototype compiles and runs on ≥2 real devices (not emulator-only — front camera + live-stream face tracking needs real hardware)
• Toggle exists to switch between MediaPipe and ARCore tracking paths for comparison
• Informal feel/latency/jitter assessment recorded (doesn't need to be rigorous, just a documented judgment call)
• Findings (worth pursuing further vs. not) reported back on parent #629

Deferred (only if we proceed)

• Removing the ARCore path / FaceTrackingManager rewiring
• Blendshape-driven gestures (blink-to-click, etc.)
• Test coverage, work-log doc

Links

Part of #629