ffi 1.22: lock in undef shape for #jni_call(*void)

Last return-type variant in the matrix. JNI's jobject is a pointer
(LocalRef) — sx's `*void` maps to LLVM `ptr` directly. CallObjectMethod
is at vtable slot 34. IR snapshot captures today's `ret ptr undef`.
Next commit adds the `.ptr => Jni.CallObjectMethod` arm.

LocalRef lifetime: the returned jobject is a JNI LocalRef bounded by
the native frame. Chains of calls within one frame consume LocalRefs
inline; calls that need to escape the frame should be promoted via
`NewGlobalRef` (already wired in the slot-interning path). Step 1.22
doesn't introduce automatic cleanup — chess use consumes objects
inline, matching the pattern in sx_android_jni.c.

examples/ffi-jni-call-08-jobject-return.sx

@@ -0,0 +1,26 @@
+// Phase 1 step 1.22 (PLAN-FFI.md): `#jni_call(*void)` (jobject
+// return). Last return-type variant in the matrix. The returned
+// jobject is a JNI LocalRef — its lifetime is bounded by the native
+// frame, so chains of calls within one frame don't need explicit
+// cleanup, but calls that escape (cached cross-frame) should be
+// promoted via `NewGlobalRef`. For chess Android use the returned
+// jobject is consumed inline by the next `#jni_call`, so no cleanup
+// is needed here.
+//
+// CallObjectMethod lives at vtable slot 34.
+
+#import "modules/std.sx";
+
+g_should_call : bool = false;
+
+get_window :: (env: *void, activity: *void) -> *void {
+    #jni_call(*void)(env, activity, "getWindow", "()Landroid/view/Window;");
+}
+
+main :: () -> s32 {
+    if g_should_call {
+        _ := get_window(null, null);
+    }
+    print("ok\n");
+    0;
+}