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/02-android-opengl-es.patch

@@ -1,19 +1,30 @@
---- a/CMakeLists.txt
-+++ b/CMakeLists.txt
-@@ -46,9 +46,17 @@
-
+diff -ru a/CMakeLists.txt b/CMakeLists.txt
+--- a/CMakeLists.txt	2026-07-31 14:04:53.195646458 +0200
++++ b/CMakeLists.txt	2026-07-31 14:05:05.703429751 +0200
+@@ -46,9 +46,27 @@
+ 
  add_compile_options(-fsigned-char -fno-strict-aliasing)
-
+ 
 -# Find OpenGL
 +# Find OpenGL. Android has no desktop GL/GLX for CMake's FindOpenGL module
-+# to find - link the prebuilt GL4ES instead (translates this file's
++# to find - build GL4ES from source instead (translates this file's
 +# desktop-style GL calls into GLES/EGL; it links GLESv2/EGL itself, so
-+# the engine doesn't need to link them directly).
++# the engine doesn't need to link them directly), via add_subdirectory
++# rather than a prebuilt .so: ANDROID_GL4ES_DIR (passed in by
++# android/app/build.gradle, same as ANDROID_PREBUILT_DIR) is a scratch,
++# patched copy of android/gl4es-src/ staged by
++# build-image/prepare-android-project.sh (mirrors how this file itself is
++# staged/patched before Gradle ever sees it) - building it as part of this
++# same CMake configure means Gradle's own incremental CMake/Ninja build
++# only recompiles it when gl4es-src/gl4es-patches actually changed, same
++# as this file, instead of needing a full build-image rebuild for every
++# gl4es-patches change.
  if(ENABLE_OPENGL)
 -	find_package(OpenGL REQUIRED)
 +	if(ANDROID)
-+		set(OPENGL_INCLUDE_DIRS ${ANDROID_PREBUILT_DIR}/gl4es/include)
-+		set(OPENGL_LIBRARIES ${ANDROID_PREBUILT_DIR}/gl4es/lib/libGL.so)
++		add_subdirectory(${ANDROID_GL4ES_DIR} ${CMAKE_BINARY_DIR}/gl4es-build)
++		set(OPENGL_INCLUDE_DIRS ${ANDROID_GL4ES_DIR}/include)
++		set(OPENGL_LIBRARIES GL)
 +	else()
 +		find_package(OpenGL REQUIRED)
 +	endif()