Add controller input and a laser-pointer-driven menu quad

- xr_input.c/h: an OpenXR action set (aim pose, trigger/select click, a
  menu-toggle button) plus ray/quad hit-testing, and a visible
  laser-pointer line drawn from the controller toward the hit point.
- xr_menu.c/h + MenuOverlay.java: a second composition-layer quad,
  toggled by a controller button, showing an off-screen, never-attached
  Android View tree (one "Keyboard" button so far) driven by synthetic
  touch events computed from the laser ray's hit UV.
- Migrate gl4es from a prebuilt binary baked into the Docker build-image
  to a vendored source snapshot (android/gl4es-src/) compiled from
  source at APK build time via CMake add_subdirectory(), patched through
  a new android/gl4es-patches/ (mirrors the existing engine-patches/
  pattern). This was needed to track down and fix a bug hit while
  building the menu: gl4es's fixed-pipeline emulation (fpe.c) was
  unconditionally substituting its own shader onto the menu's draw call
  (fpe_ReleventState() always sets alphafunc to a nonzero sentinel, so
  its fpe_IsEmpty() check could never see the state as empty), making
  the menu quad show the game's own rendering instead of its own
  content. Fixed by routing the menu's blit through a real
  glBlitFramebuffer() call instead of a shader-based draw, which gl4es's
  fpe.c has nothing to intercept - documented in README.md's new
  "Debugging notes" section.
- Renumber android/engine-patches/ to close the gap left by removing an
  unrelated diagnostic-only patch, and strip investigation-journal
  comments and dead diagnostic code (temporary tracing, env-var probes)
  left over from finding the bug above.
- Restructure README.md into numbered sections, document every
  engine/gl4es patch, add Android Studio dev/testing-cycle instructions,
  and generalize Quest-specific wording to any OpenXR headset. Update
  NOTICE.txt to match (gl4es is now vendored/patched source, not a
  prebuilt binary; add the OpenXR-SDK loader).

android/engine-patches/10-android-openxr-cmake.patch

@@ -0,0 +1,39 @@
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -64,6 +64,19 @@
+ 	endif(WIN32)
+ endif(ENABLE_OPENGL)
+ 
++# OpenXR loader (Android only for now - see android/app/src/main/cpp/
++# xr_session.c, which submits the game's existing render as a composition
++# layer instead of presenting to a normal window/surface). EGL is linked
++# explicitly too - xr_session.c calls eglGetCurrentDisplay/Context/
++# QueryContext/ChooseConfig directly (to share the GL context gl4es/SDL
++# already made), and unlike GL itself (where gl4es's own libGL.so already
++# provides every symbol the engine or xr_session.c call), there's no other
++# EGL provider in this link.
++if(ANDROID)
++	set(OPENXR_INCLUDE_DIRS ${ANDROID_PREBUILT_DIR}/openxr/include)
++	set(OPENXR_LIBRARIES ${ANDROID_PREBUILT_DIR}/openxr/lib/libopenxr_loader.so EGL)
++endif(ANDROID)
++
+ if(ENABLE_SDL2 MATCHES "ON")
+ 	find_package(SDL2 REQUIRED)
+ 	if(SDL2_FOUND)
+@@ -109,6 +122,8 @@
+ 	${SDL2_MIXER_INCLUDE_DIRS}
+ 	${FLUIDSYNTH_INCLUDE_DIRS}
+ 	${OPENGL_INCLUDE_DIRS}
++	${OPENXR_INCLUDE_DIRS}
++	${ANDROID_EXTRA_INCLUDE_DIRS}
+ )
+ 
+ if(NOT WIN32)
+@@ -439,6 +454,7 @@
+ 	${SDL2_MIXER_LIBRARIES}
+ 	${FLUIDSYNTH_LIBRARIES}
+ 	${OPENGL_LIBRARIES}
++	${OPENXR_LIBRARIES}
+ 	${ALSA_LIBRARIES}
+ 	$<$<BOOL:${ANDROID}>:log>
+ )