Pose the cockpit hands from the headset's hand tracking

- New [vr] hand_tracking (default off, Quest only for now): the Quest launcher's Settings > VR and
  the headset panel's VR tab, under hand steering. Two XR_EXT_hand_tracking trackers are located
  every XR frame: with the controllers held the Quest builds the joints from their touch sensors
  (XR_EXT_hand_tracking_data_source's controller source), once they are put down from its
  cameras. The trackers exist only while the option and hand steering are on and also serve the
  runtime hand mesh; the extensions (plus XR_FB_hand_tracking_aim) are asked for when either is
  on at launch.
- Aurora skins the runtime mesh with the joints themselves (tracked pose times inverse bind pose,
  no curl, no grip); runtimes with joints but no mesh get a skeleton; non-finite joints put only
  that hand back on its grip curl. AuroraCockpitHand carries the 26 seated-frame joints and radii.
- The manifest declares horizonos.permission.HAND_TRACKING (and the deprecated
  com.oculus.permission.HAND_TRACKING), both normal permissions with no prompt on a Quest 3, and
  oculus.software.handtracking as optional; without it Horizon OS keeps the app controllers-only.
  Bare hands then drive khr/simple_controller, so on Android a hand whose squeeze action is
  inactive and select active is treated as bare: with the option off it presses nothing but the
  menu gesture, is not drawn and feeds no Wii Remote motion.
- Interaction-profile changes and tracker sources are logged. The pure rules (grasp from finger
  flexion, bare latch, pinch gate, bare-hand buttons, flick) live in the OpenXR-free
  vr/openxr_hand_tracking.h with mkw_vr_hand_tracking_tests; the driving and flick parts are
  wired in the next commits. Docs: OPENXR.md, docs/quest-port.md, README.

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

runtime/CMakeLists.txt

@@ -415,8 +415,10 @@ add_test(NAME mkw_vr_first_person_tests COMMAND mkw_vr_first_person_tests)
 
 # The first-person cockpit (seated eye, wheel and handlebar geometry, level seat,
 # native wheel vertices) and hand steering (grab, turn, hand-off to the game),
-# ported from heurazy's mario-kart-wii-VR-port. All header-only.
-foreach(test_name mkw_steering_wheel_tests mkw_vr_cockpit_tests mkw_vr_hand_steering_tests mkw_vr_camera_toggle_tests)
+# ported from heurazy's mario-kart-wii-VR-port, and tracked hands (grasp, bare-hand
+# buttons, flick). All header-only.
+foreach(test_name mkw_steering_wheel_tests mkw_vr_cockpit_tests mkw_vr_hand_steering_tests mkw_vr_camera_toggle_tests
+                  mkw_vr_hand_tracking_tests)
     string(REGEX REPLACE "^mkw_" "" test_source "${test_name}")
     add_executable(${test_name} "${CMAKE_CURRENT_LIST_DIR}/tests/${test_source}.cpp")
     target_include_directories(${test_name} PRIVATE "${CMAKE_CURRENT_LIST_DIR}/include")