Add OpenXR bring-up: render the game as a floating quad in an immersive Quest session

- New android/app/src/main/cpp/xr_session.c (+.h): owns the OpenXR
  instance/session/local-space/swapchain and the per-frame
  xrWaitFrame/xrBeginFrame/xrEndFrame loop, submitting the game's
  existing flat render as a single head-tracked XrCompositionLayerQuad.
  No stereo rendering or controller input yet - that's steps B/C/D of
  the plan.
- 11-android-openxr-cmake.patch: links the OpenXR loader as a build
  dependency, staged into jniLibs the same way as gl4es/SDL2.
  build-image/Dockerfile and prepare-android-project.sh build and stage
  it.
- 12-android-openxr-present.patch: redirects OpenGL.cc's final present
  step (opengl_swap_and_restore / SDLDraw's software path) into the XR
  swapchain FBO instead of the window, on Android only. Two real gl4es
  bugs had to be worked around to get pixels on screen at all:
    - gl4es's own glBindFramebuffer errors on an FBO id it didn't create
      itself, even though the real driver-level bind succeeds - fixed by
      creating the swapchain FBO container via gl4es's own
      glGenFramebuffers/glBindFramebuffer, and only using the real
      (dlsym'd) driver call for attaching OpenXR's foreign swapchain
      texture, which gl4es's own attach can't handle.
    - gl4es's glBegin/glVertexAttrib/glVertex3f immediate-mode emulation
      only captures a fresh per-vertex value for attribute 0 (position,
      driven directly by glVertex3f) - custom attributes like this
      shader's texcoords/light are GLES2's *constant*-attribute API and
      applied once for the whole draw, not per vertex, silently
      collapsing the UI-overlay quad to a single sampled texel. Fixed by
      switching that one draw call to real vertex arrays
      (glVertexAttribPointer/glDrawArrays).
  Also flips the V texcoord to match SDL's top-down row order against
  GL's texture convention, and reuses a persistent texture object
  instead of a fresh gen/upload/delete every frame.
- AndroidManifest.xml: declares the immersive-HMD intent category and
  focus-aware metadata, drops the 2D-panel layout hint.
- README: documents the new OpenXR immersive mode.

android/app/build.gradle

@@ -115,15 +115,23 @@ android {
                 // build-image/Dockerfile's ANDROID_16KB_LDFLAGS, which does
                 // the same for the prebuilt SDL2/SDL2_mixer/fluidsynth-lite/
                 // gl4es .so's this links against.
+                // ANDROID_EXTRA_SOURCES is a semicolon-separated CMake list,
+                // not shell-split - AGP passes each "arguments" string
+                // straight through as one argv element to cmake, so the
+                // ';' below survives intact. ANDROID_EXTRA_INCLUDE_DIRS lets
+                // engine/'s own OpenGL.cc find "xr_session.h" (see
+                // android/engine-patches/11-android-openxr-cmake.patch,
+                // which adds it to include_directories()).
                 arguments "-DENABLE_SDL2=ON", "-DENABLE_SOUND=BUNDLED", "-DENABLE_FLUIDSYNTH=BUNDLED", \
                     "-DANDROID_PREBUILT_DIR=${prebuiltDir}", \
+                    "-DANDROID_EXTRA_INCLUDE_DIRS=${projectDir}/src/main/cpp", \
                     "-DCMAKE_PREFIX_PATH=${prebuiltDir}/sdl2", \
                     "-DCMAKE_FIND_ROOT_PATH=${prebuiltDir}/sdl2", \
                     "-DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=BOTH", \
                     "-DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=BOTH", \
                     "-DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=BOTH", \
                     "-DCMAKE_SHARED_LINKER_FLAGS=-Wl,-z,max-page-size=16384", \
-                    "-DANDROID_EXTRA_SOURCES=${projectDir}/src/main/cpp/questshock_native.c"
+                    "-DANDROID_EXTRA_SOURCES=${projectDir}/src/main/cpp/questshock_native.c;${projectDir}/src/main/cpp/xr_session.c"
                 abiFilters 'arm64-v8a'
             }
         }