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.

examples/99-android-egl-clear.sx

@@ -1,79 +0,0 @@
-// Android-only: pure-sx NativeActivity that brings up EGL on the
-// ANativeWindow delivered by native_app_glue and clears the screen
-// every frame via GLES3. Equivalent of `examples/63-metal-clear.sx`
-// for the Android target.
-//
-// Entry-point contract (the "via Platform" shape):
-//   - User writes BOTH `main` and `android_main` at top level.
-//   - `android_main(app)` calls `sx_android_bootstrap(app)` and then
-//     `main()`. The library never names `main`; the OS-shape entry
-//     symbol lives in user code, where the other entry symbols are.
-//   - `main` instantiates `AndroidPlatform`, calls `init`, then
-//     `run_frame_loop` which drives the looper until destroyRequested.
-//
-// This exercises end-to-end the Android pipeline shipped in Session 70:
-//   - `sx build --target android` toolchain (NDK clang + glue link).
-//   - `--apk` APK assembly (manifest + aapt2 + zipalign + apksigner).
-//   - `android_main` (user-written here) gets external LLVM linkage
-//     via the `isExportedEntryName` allowlist in lower.zig.
-//   - `AndroidPlatform.run_frame_loop` drains ALooper events,
-//     creates EGL on `APP_CMD_INIT_WINDOW`, ticks the closure every
-//     16 ms.
-//
-// Build + install on a connected device:
-//
-//   /Users/agra/projects/sx/zig-out/bin/sx build --target android \
-//       --apk /tmp/sxhello.apk --bundle-id co.swipelab.sxhello \
-//       -o /tmp/libsxhello.so examples/99-android-egl-clear.sx
-//   adb install -r /tmp/sxhello.apk
-//   adb shell am start -n co.swipelab.sxhello/android.app.NativeActivity
-//   adb logcat -d --pid=$(adb shell pidof co.swipelab.sxhello)
-//
-// Expected: solid purple frame on the device. Periodic
-// `rendered 60 frames` lines in logcat.
-
-#import "modules/std.sx";
-#import "modules/compiler.sx";
-#import "modules/platform/api.sx";
-#import "modules/platform/android.sx";
-
-// GLES3 (linked via -lGLESv3)
-glClearColor :: (r: f32, g: f32, b: f32, a: f32) #foreign;
-glClear      :: (mask: u32) #foreign;
-GL_COLOR_BUFFER_BIT :u32: 0x4000;
-
-frame_count : s32 = 0;
-g_plat      : *AndroidPlatform = null;
-
-frame_tick :: () {
-    fc := g_plat.begin_frame();
-    glClearColor(0.5, 0.2, 0.8, 1.0);  // purple
-    glClear(GL_COLOR_BUFFER_BIT);
-    g_plat.end_frame();
-
-    frame_count += 1;
-    if (frame_count % 60) == 0 {
-        __android_log_print(4, "sxhello".ptr, "rendered 60 frames\n".ptr);
-    }
-}
-
-main :: () -> s32 {
-    inline if OS == .android {
-        plat : AndroidPlatform = .{};
-        plat.init("sxhello", 0, 0);
-        g_plat = @plat;
-        plat.run_frame_loop(() => frame_tick());
-    }
-    0;
-}
-
-// OS-shape entry symbol. native_app_glue's
-// `ANativeActivity_onCreate` ultimately calls this on the worker
-// thread. We hand the app pointer to the platform module and then
-// let user `main` drive the normal cross-platform setup path.
-android_main :: (app: *void) {
-    inline if OS == .android {
-        sx_android_bootstrap(app);
-        main();
-    }
-}