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).

build-image/build-apk.sh

@@ -1,9 +1,9 @@
 #!/usr/bin/env bash
 # Builds the Quest APK - see build-image/prepare-android-project.sh (which
-# this calls) for how android/ gets a scratch, patched copy of engine/,
-# the Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es prebuilts, and bundled
-# assets staged in. Run via ../run-image.sh (or directly, if already
-# inside this image - see the Makefile's `apk` target).
+# this calls) for how android/ gets scratch, patched copies of engine/ and
+# gl4es-src/, the Android SDL2/SDL2_mixer/fluidsynth-lite/openxr prebuilts,
+# and bundled assets staged in. Run via ../run-image.sh (or directly, if
+# already inside this image - see the Makefile's `apk` target).
 #
 # Unlike build-engine.sh, this step needs network access: Gradle/AGP's
 # own dependency resolution isn't prebuilt into the image (see