ffi #jni_main R.4: require #jni_main on Android; retire android_main acceptance

`checkRequiredEntryPoints` no longer accepts `android_main` as an
Android entry — `#jni_main #jni_class("...")` is now the sole accepted
path. The diagnostic walks the user through declaring an Activity +
Bundle foreign decl. `isExportedEntryName` drops `android_main` and
`ANativeActivity_onCreate` (both were for the legacy NativeActivity
glue path R.2 stopped linking by default).

Migrates the two cross-compile examples that previously carried an
`android_main` trampoline to a minimal `#jni_main #jni_class(...) { }`
stub:

  - `examples/ffi-jni-call-02-void.sx` — tests `#jni_call(void)` lowering
  - `examples/ffi-objc-call-10-os-gate.sx` — tests `inline if OS` gating

Both stubs are empty (no `onCreate` body) so they exercise the
entry-point check + R.3's JNI-symbol synthesis pass produces no
symbols. 131 host / 4 cross / zig build test all green.

`examples/99-android-egl-clear.sx` still uses `android_main` and the
AndroidPlatform/native_app_glue stack — its Android-target build now
fails the entry-point check. R.5 removes it along with the rest of
the legacy NativeActivity surface.

examples/ffi-objc-call-10-os-gate.sx

@@ -13,6 +13,11 @@
 #import "modules/std.sx";
 #import "modules/compiler.sx";
 
+// Empty stub class — Android cross-compile requires a `#jni_main`
+// declaration to satisfy the entry-point check. This file is testing
+// `inline if OS` gating around `#objc_call`, not Activity wiring.
+SxObjcOsGateStub :: #jni_main #jni_class("co/swipelab/sxobjcosgate/SxObjcOsGateStub") { }
+
 main :: () -> s32 {
     inline if OS == {
         case .ios: {
@@ -32,12 +37,3 @@ main :: () -> s32 {
     }
     0;
 }
-
-// Android target requires `android_main` as the NDK entry — kept as
-// a 3-line trampoline so this example can pass through
-// `--target android` builds in `tests/cross_compile.sh`.
-android_main :: (app: *void) {
-    inline if OS == .android {
-        main();
-    }
-}