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).

.gitea/workflows/build.yml

@@ -45,7 +45,11 @@ jobs:
           command -v cmake >/dev/null 2>&1 || { echo "PREFLIGHT FAIL: cmake not in PATH" >&2; exit 1; }
           [ -n "${QUESTSHOCK_BUILD_IMAGE:-}" ] || { echo "PREFLIGHT FAIL: QUESTSHOCK_BUILD_IMAGE not set - not running inside the questshock build-image?" >&2; exit 1; }
           [ -d /opt/prebuilt/built_sdl ] || { echo "PREFLIGHT FAIL: /opt/prebuilt/built_sdl missing" >&2; exit 1; }
-          [ -d /opt/prebuilt/android/gl4es ] || { echo "PREFLIGHT FAIL: /opt/prebuilt/android/gl4es missing - runner is using a build-image older than the Android/Quest layer" >&2; exit 1; }
+          # gl4es itself is no longer prebuilt here (compiled from source at
+          # APK-build time instead - see build-image/Dockerfile's gl4es
+          # comment) - openxr was added to this image in the same
+          # Android/Quest layer, so it's an equally good marker of that.
+          [ -d /opt/prebuilt/android/openxr ] || { echo "PREFLIGHT FAIL: /opt/prebuilt/android/openxr missing - runner is using a build-image older than the Android/Quest layer" >&2; exit 1; }
 
       - name: Build package
         run: make package