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.zig

@@ -43,7 +43,10 @@ pub const Options = struct {
     /// Used to resolve `*Foo` cross-class refs in method signatures.
     classes: ?*const std.StringHashMap([]const u8) = null,
     /// Default superclass when the user doesn't write `#extends ...;`.
-    default_extends: []const u8 = "android.app.NativeActivity",
+    /// `android.app.Activity` is the standard base for Java-driven
+    /// Activities — `NativeActivity` is the legacy NDK path that
+    /// requires native_app_glue's `ANativeActivity_onCreate`.
+    default_extends: []const u8 = "android.app.Activity",
 };
 
 /// Emit a `.java` source for the given foreign-class decl. Result is