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-jni-call-02-void.sx

@@ -21,14 +21,17 @@
 #import "modules/std.sx";
 #import "modules/compiler.sx";
 
+// Android target requires a `#jni_main` Activity declaration to
+// satisfy the entry-point check. An empty stub class is enough — this
+// file is testing `#jni_call` lowering, not Activity wiring.
+SxJniCallVoidStub :: #jni_main #jni_class("co/swipelab/sxjnicall/SxJniCallVoidStub") { }
+
 main :: () -> s32 {
     inline if OS == .android {
-        // Real Android entry passes env + target via android_main /
+        // Real Android entry passes env + target via the user's `onCreate`
-        // ANativeActivity (modules/platform/android.sx). For the cross-
+        // / `#jni_attach`. For the cross-compile-only test we just need
-        // compile-only test we just need the lowering to emit valid
+        // the lowering to emit valid IR; runtime correctness is exercised
-        // IR; runtime correctness is exercised by the chess
+        // by the chess sx_android_query_safe_insets path.
-        // sx_android_query_safe_insets path once Phase 1D for
-        // sx_android_jni.c lands.
         env    : *void = null;
         target : *void = null;
         #jni_env(env) {
@@ -40,12 +43,3 @@ 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();
-    }
-}

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();
-    }
-}

src/ir/lower.zig

@@ -28,8 +28,6 @@ const Builder = mod_mod.Builder;
 /// runtime resolves by name mangling — same rule.
 fn isExportedEntryName(name: []const u8) bool {
     return std.mem.eql(u8, name, "main") or
-        std.mem.eql(u8, name, "android_main") or
-        std.mem.eql(u8, name, "ANativeActivity_onCreate") or
         std.mem.eql(u8, name, "JNI_OnLoad") or
         std.mem.startsWith(u8, name, "Java_");
 }
@@ -231,18 +229,12 @@ pub const Lowering = struct {
         self.synthesizeJniMainStubs();
     }
 
-    /// On Android, the OS loads the .so via one of two entry paths:
+    /// On Android, the OS loads the .so via a Java-side Activity declared
-    ///
+    /// with `#jni_main #jni_class("...")`. The Java class drives the
-    ///   1. **`#jni_main` Activity** (preferred) — Java-side Activity class
+    /// lifecycle (onCreate / onPause / etc.) and sx provides the native
-    ///      declared as `Foo :: #jni_main #jni_class("...") { onCreate :: ... }`.
+    /// delegates bound via JNI name mangling. Without a `#jni_main` decl
-    ///      The Java class drives lifecycle; sx provides native delegates.
+    /// there's no entry point — the .so would load but Android has nothing
-    ///   2. **`android_main` trampoline** (legacy NativeActivity path) — user
+    /// to call into.
-    ///      defines `android_main(app: *void)`, native_app_glue's
-    ///      `ANativeActivity_onCreate` jumps into it on a worker thread.
-    ///
-    /// Either satisfies the check. When both are missing, the .so won't
-    /// have an entry point Android can reach, so emit a diagnostic that
-    /// shows both options.
     fn checkRequiredEntryPoints(self: *Lowering) void {
         const tc = self.target_config orelse return;
         if (!tc.isAndroid()) return;
@@ -253,23 +245,15 @@ pub const Lowering = struct {
             if (fcd.is_main and !fcd.is_foreign and fcd.runtime == .jni_class) return;
         }
 
-        const wanted = self.module.types.internString("android_main");
-        for (self.module.functions.items) |func| {
-            if (func.name != wanted) continue;
-            if (func.is_extern) continue;
-            if (func.blocks.items.len == 0) continue;
-            return;
-        }
-
         if (self.diagnostics) |diags| {
             diags.addFmt(.err, null,
-                "target is Android but no entry point declared. " ++
+                "target is Android but no `#jni_main` Activity declared. " ++
-                "Either declare a `#jni_main` Activity class:\n\n" ++
+                "The OS launches a Java-side Activity that delegates lifecycle " ++
+                "callbacks into sx — declare one like:\n\n" ++
+                "    Bundle :: #foreign #jni_class(\"android/os/Bundle\") {{ }}\n\n" ++
                 "    MyApp :: #jni_main #jni_class(\"co/example/MyApp\") {{\n" ++
-                "        onCreate :: (self: *Self, b: *Bundle) {{ /* ... */ }};\n" ++
+                "        onCreate :: (self: *Self, b: *Bundle) {{ /* ... */ }}\n" ++
-                "    }}\n\n" ++
+                "    }}", .{});
-                "or define an `android_main` trampoline that bootstraps the " ++
-                "legacy NativeActivity path (see `examples/99-android-egl-clear.sx`).", .{});
         }
     }