ffi #jni_main R.1: manifest synthesis + default parent → android.app.Activity

When `Compilation.lowering_jni_main_decls` is non-empty, `createApk`
synthesises a manifest whose `<activity android:name>` points at the
user's `#jni_main` class (dotted form of the foreign path), sets
`android:hasCode="true"` so Android loads the bundled classes.dex, and
drops the `android.app.lib_name` meta-data (that's the NativeActivity-
specific autoload mechanism — Java-driven Activities load the .so via
`System.loadLibrary` from a Java static initializer slice R.3 will
emit). The legacy NativeActivity path stays as the fallback when no
`#jni_main` decl is present.

`jni_java_emit.zig`'s default superclass moves from
`android.app.NativeActivity` to `android.app.Activity` — the former
requires native_app_glue's `ANativeActivity_onCreate` to be in the .so,
which the next slice (R.2) will stop linking by default.

Verified end-to-end on the slice 2 smoke APK: `aapt2 dump xmltree`
shows `android:name="co.swipelab.sxjnimain.SxApp"` + `hasCode="true"`,
and `dexdump -l plain` confirms SxApp now extends `Landroid/app/Activity;`.
99-android-egl-clear's APK still uses the NativeActivity manifest as
before (legacy path intact for R.2-R.5).

src/ir/jni_java_emit.test.zig

@@ -50,7 +50,7 @@ test "rejects non-main decl" {
     try std.testing.expectError(emit.EmitError.NotAJniMainClass, result);
 }
 
-test "void onCreate(Bundle) with #extends NativeActivity default" {
+test "void onCreate(Bundle) with default Activity superclass" {
     const a = std.testing.allocator;
     var arena = std.heap.ArenaAllocator.init(a);
     defer arena.deinit();
@@ -85,7 +85,7 @@ test "void onCreate(Bundle) with #extends NativeActivity default" {
     const expected =
         \\package co.swipelab.sx_runtime;
         \\
-        \\public class SxNativeActivity extends android.app.NativeActivity {
+        \\public class SxNativeActivity extends android.app.Activity {
         \\    @Override
         \\    public void onCreate(android.os.Bundle b) {
         \\        super.onCreate(b);