ffi #jni_main R.5: retire legacy NativeActivity surface

Deletes the entire NativeActivity / native_app_glue / ALooper stack
that the previous Android entry path was built around:

  - `examples/99-android-egl-clear.sx` — the demo of the legacy path.
  - `library/modules/platform/android.sx` — `AndroidPlatform.init`,
    `run_frame_loop`, `sx_android_bootstrap`, `g_android_app`, plus
    the ALooper / AInputEvent / ANativeActivity / AConfiguration
    foreign decls that fed them. The JNI helpers (`sx_load_javavm_fn`,
    `sx_android_get_env`, `sx_query_safe_insets_jni`, the
    `ANATIVEACTIVITY_*` offsets) were tied to the ANativeActivity*
    delivered to `android_main` — they're stale now that the OS hands
    sx code a Java Activity directly via `onCreate(JNIEnv*, jobject)`.
  - `library/vendors/sx_android_jni/sx_android_jni.c` — the input-
    handler installer (`sx_android_install_input_handler`), which
    poked NDK app-pointer field offsets that no longer exist.

`library/modules/platform/android_jni.sx` (the `Activity`/`Window`/
`View`/`WindowInsets` `#jni_class` registry used for safe-insets
dispatch) survives — it's standalone declarative bindings useful from
any `#jni_main` onCreate body. Docstring updated to drop the
"imported from android.sx" framing.

131 host / 4 cross / zig build test all green. End-to-end smoke APK
still produces the expected JNI-mangled symbol +
SxApp-extends-Activity dex.

External consumers (chess) will need to migrate their entry from the
`AndroidPlatform.run_frame_loop` model to the `#jni_main` model
(Java-side Activity drives lifecycle; onSurfaceChanged / Choreographer
drive frames via JNI callbacks). That migration is downstream work.

library/modules/gpu/gles3.sx

@@ -28,7 +28,8 @@
 
 // EGL bootstrap helper — used to populate opengl.sx's fn pointers once
 // the EGL context is current. We declare only the loader here; EGL
-// surface/context creation lives in platform/android.sx.
+// surface/context creation belongs in the consumer's `#jni_main`
+// Activity (e.g. an `onCreate` body that calls `eglCreateContext`).
 eglGetProcAddress :: (name: [*]u8) -> *void #foreign;