Hand a put-down controller's side to the cameras

- Started with the controllers connected, a race stayed on them: Horizon OS switches all input
  between controllers and hands, and back to the controllers as soon as one lying on a table moves
  (a Quest 3 log went touch_controller, simple_controller, touch_controller within seconds). An
  app cannot disconnect the controllers.
- With tracked hands on, the input now resumes XR_META_simultaneous_hands_and_controllers (Meta's
  multimodal), which overrides that switching: a controller not in a hand no longer owns it, so the
  cameras track that hand at once, and a held controller keeps working. Paused again when the
  option goes off; a refused resume is logged and not retried until the option is toggled.
- A hand is bare when its squeeze action is inactive and it either drives khr/simple_controller or
  has camera-tracked joints, since a free hand under simultaneous tracking may get a profile our
  actions are not bound in. Docs: OPENXR.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/include/vr/openxr_input.h

@@ -161,6 +161,7 @@ private:
     // settings ask for them, and both hands located for `time`, their joints
     // in `seat` when it is valid.
     void LoadHandTracking();
+    void UpdateSimultaneousHandsAndControllers(bool wanted);
     void UpdateHandTrackers();
     void DestroyHandTrackers();
     void LocateHands(XrTime time, const driving::SeatFrame& seat);
@@ -225,6 +226,12 @@ private:
     PFN_xrLocateHandJointsEXT m_locate_hand_joints = nullptr;
     bool m_hand_data_source = false; // XR_EXT_hand_tracking_data_source
     bool m_hand_aim = false;         // XR_FB_hand_tracking_aim
+    // XR_META_simultaneous_hands_and_controllers: resumed while tracked hands
+    // are on, so a controller put down gives its hand to the cameras at once.
+    PFN_xrResumeSimultaneousHandsAndControllersTrackingMETA m_resume_simultaneous = nullptr;
+    PFN_xrPauseSimultaneousHandsAndControllersTrackingMETA m_pause_simultaneous = nullptr;
+    bool m_simultaneous = false;
+    bool m_simultaneous_failed = false;
     XrHandTrackerEXT m_hand_trackers[kHands]{};
     bool m_hand_trackers_failed = false;
     bool m_logged_hand_restart = false;