Drive the engine's mouse cursor from the VR aim ray on the game quad

The game quad's laser beam was purely visual - aiming at the game's own
UI (inventory, menus, dialogs) had no way to interact with it. Add a
real mouse pointer: absolute cursor position plus a single left click,
mirroring a plain point-and-click mouse (no mouselook, no right-click,
no drag).

New xr_mouse.c/h bridges the aim-ray hit-test to the vendored engine's
own public mouse API - mouse_put_xy() for position, a synthetic
SDL_MOUSEBUTTONDOWN/UP (SDL_BUTTON_LEFT) via SDL_PushEvent for clicks,
the same technique nativeSendPrintableChar() already uses for
synthetic keyboard input. No engine patch needed. Deliberately bypasses
SDLActivity.onNativeMouse()/SDL_MOUSEMOTION/SetMouseXY(), whose
physical-window-size scaling is unreliable in this headless immersive
build.

xr_input_try_game_quad() now reports the hit u,v; a new
g_game_touch_active[] tracks a held click on the game quad so it
survives the ray straying onto the keyboard/menu before release,
gating those overlays' claims the same way pending overlay touches
already do.

android/app/src/main/cpp/xr_session.c

@@ -469,3 +469,8 @@ void xr_get_game_quad_extent(float *distance_m, float *half_width_m, float *half
     *half_height_m =
         QUAD_WIDTH_METERS * 0.5f * (float)g_game_swapchain.height / (float)g_game_swapchain.width;
 }
+
+void xr_get_game_resolution(int *width, int *height) {
+    *width = g_game_swapchain.width;
+    *height = g_game_swapchain.height;
+}