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_mouse.h

@@ -0,0 +1,35 @@
+// Bridges VR aim-ray hit-testing (xr_input.c) to the vendored engine's
+// mouse input (engine/src/Libraries/INPUT/Source/mouse.h) - lets the
+// controller's laser pointer drive the game's own UI (inventory, menus,
+// dialogs) exactly like a desktop mouse: absolute cursor position plus a
+// single left button, no relative/mouselook mode, no right button.
+#ifndef QUESTSHOCK_XR_MOUSE_H
+#define QUESTSHOCK_XR_MOUSE_H
+
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Sets the engine's absolute mouse position (mouse_put_xy()) - call every
+// frame the aim ray is actually hitting the game quad; x/y are pixel
+// coordinates in the game's own logical resolution (see
+// xr_get_game_resolution() in xr_session.h), already clamped by the
+// caller. Do not call on frames the ray isn't hitting the game quad - the
+// cursor should hold its last position, not snap elsewhere.
+void xr_mouse_move(int x, int y);
+
+// Pushes a synthetic SDL_MOUSEBUTTONDOWN/UP (SDL_BUTTON_LEFT) - matches
+// KeyboardOverlay's nativeSendPrintableChar() in questshock_native.c: a
+// plain SDL_Event built by hand and handed to the public SDL_PushEvent(),
+// no engine patch needed. pump_events() (sdl_events.c) reads whatever
+// mouse_put_xy() last set, not any x/y carried on the event itself - call
+// xr_mouse_move() first if position needs updating this frame.
+void xr_mouse_click(bool down);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif