ffi 1.20: lock in undef shape for #jni_call(f64)

Test-add for the jdouble return-type variant — `#jni_call(f64)(env,
target, "getValue", "()D")`. First non-integer return type for JNI.
IR snapshot captures today's `ret double undef` placeholder. The
next commit adds the `.f64 => Jni.CallDoubleMethod` arm.

examples/ffi-jni-call-06-jdouble-return.sx

@@ -0,0 +1,19 @@
+// Phase 1 step 1.20 (PLAN-FFI.md): `#jni_call(f64)` (jdouble return).
+// First non-integer return type for JNI — exercises the LLVM
+// `double` ret path through CallDoubleMethod (vtable slot 58).
+
+#import "modules/std.sx";
+
+g_should_call : bool = false;
+
+read_double :: (env: *void, target: *void) -> f64 {
+    #jni_call(f64)(env, target, "getValue", "()D");
+}
+
+main :: () -> s32 {
+    if g_should_call {
+        _ := read_double(null, null);
+    }
+    print("ok\n");
+    0;
+}