ffi 1.19: lock in undef shape for #jni_call(s64)

Test-add for the jlong return-type variant — same shape as 1.18's
jint test but exercising `#jni_call(s64)(env, target,
"currentTimeMillis", "()J")`. Today the non-void switch falls
through to `LLVMGetUndef`; the IR snapshot captures the placeholder.

The next commit adds the `.s64 => Jni.CallLongMethod` arm. The
snapshot will update to show the full dispatch through vtable slot
52, reusing the 1.17 slot interning machinery.

examples/ffi-jni-call-05-jlong-return.sx

@@ -0,0 +1,22 @@
+// Phase 1 step 1.19 (PLAN-FFI.md): `#jni_call(s64)` (jlong return).
+// Today the non-void switch falls through to `LLVMGetUndef`; the IR
+// snapshot captures the placeholder. The make-green commit adds the
+// `.s64 => Jni.CallLongMethod` arm and the snapshot updates to show
+// the full dispatch through vtable slot 52, reusing the 1.17 slot
+// interning.
+
+#import "modules/std.sx";
+
+g_should_call : bool = false;
+
+read_long :: (env: *void, target: *void) -> s64 {
+    #jni_call(s64)(env, target, "currentTimeMillis", "()J");
+}
+
+main :: () -> s32 {
+    if g_should_call {
+        _ := read_long(null, null);
+    }
+    print("ok\n");
+    0;
+}