Add a Steam Frame flavour of the Android app with Frame controller bindings

The Steam Frame runs Android apps through Lepton with SteamVR's OpenXR
runtime. A third headset flavour, steamFrame, targets it:

- -mcpu=cortex-x4+nosve for the Snapdragon 8 Gen 3 (its firmware does
  not expose SVE, which clang otherwise auto-vectorises with), from one
  flavour-to-CPU map that the kit export now reads instead of guessing
  from the variant name.
- MKW_ANDROID_HEADSET=steam_frame defines MKW_HEADSET_STEAM_FRAME for
  the runtime's own targets, for Frame-specific defaults.
- A manifest without the Horizon OS entries, and FrameEntryActivity as
  the single real MAIN/LAUNCHER activity with the Khronos and Oculus VR
  categories, which Lepton needs to start an app in VR. It opens the
  setup panel and, when the selected game can start, the game on top.
- XR_VALVE_frame_controller_interaction: the Frame controller profile
  with its left D-pad (new dpad_* actions, the Wii Remote's D-pad or the
  gamepad's), View as menu and the left shoulder as the panel button.
  Also requested on Windows for SteamVR streaming to a Frame.
- Build-Quest.ps1, Build-QuestGame.ps1 and Run-Quest.ps1 take
  -Headset frame.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019HBRGKTE1GnN2ah8gcZKr3

runtime/include/vr/openxr_input.h

@@ -193,6 +193,11 @@ private:
     XrAction m_button_primary = XR_NULL_HANDLE;   // A / X
     XrAction m_button_secondary = XR_NULL_HANDLE; // B / Y
     XrAction m_menu = XR_NULL_HANDLE;
+    // The Steam Frame's left D-pad (valve/frame_controller_valve); Touch has none.
+    XrAction m_dpad_up = XR_NULL_HANDLE;
+    XrAction m_dpad_down = XR_NULL_HANDLE;
+    XrAction m_dpad_left = XR_NULL_HANDLE;
+    XrAction m_dpad_right = XR_NULL_HANDLE;
     XrAction m_aim_pose = XR_NULL_HANDLE;
     XrAction m_grip_pose = XR_NULL_HANDLE;
     XrAction m_haptic = XR_NULL_HANDLE;