test(C0.2): pin curated-bindings shape — #source-only unit + hand #foreign works (JIT+AOT), but the ref is decorative (xfail for C3.1)

examples/1620-cimport-foreign-ref-unvalidated.sx

@@ -0,0 +1,18 @@
+// Pins the C3 gap (PLAN-C C0.2b): the `#foreign` library/unit ref is
+// DECORATIVE today — `nosuchunit` names nothing anywhere, yet this
+// compiles and the symbol resolves globally (the unit's objects are in
+// the program; the ref is never consulted). After C3 this is a
+// compile-time diagnostic: a #foreign ref must name a #library or a
+// named `#import c` unit.
+#import "modules/std.sx";
+
+refs :: #import c {
+    #source "1620-cimport-foreign-ref-unvalidated/ref.c";
+};
+
+ref_answer :: () -> i32 #foreign nosuchunit "ref_answer";
+
+main :: () -> i32 {
+    print("ref = {}\n", ref_answer());
+    0
+}