ffi 1.6a: xfail — #objc_call with non-void return rejected today

102/102 regression tests pass (+ffi-objc-call-04-primitive-returns
with xfail snapshot capturing today's diagnostic).

Pinned scenario: `[NSObject class]` — `#objc_call(*void)(null, "class")`.
Should return a non-null Class pointer once the lowering supports
non-void returns. Today the Phase 1.3 restriction trips with:

  #objc_call: only `void` return + (recv, selector) is lowered today;
  non-void / arg-bearing arities land in later phase-1 steps

The next commit (1.6b) introduces an `objc_msg_send` IR opcode that
bundles (recv, sel, args, ret_ty) and emit_llvm builds a per-call-
site LLVM function type, sharing one declared `@objc_msgSend`
symbol across return-type variants. Five primitive returns
(*void / bool / s32 / s64 / f64) get folded in across 1.6b–c.

examples/ffi-objc-call-04-primitive-returns.sx

@@ -0,0 +1,31 @@
+// Phase 1 step 1.6 (PLAN-FFI.md): non-void return shapes through
+// `#objc_call`. The Phase 1.3 lowering hardcoded void+2-arg only;
+// this test exercises five primitive return types via real
+// Foundation classes:
+//
+//   *void  — [NSObject class] returns Class (ptr)
+//   bool   — [NSObject isMemberOfClass: cls] returns BOOL
+//   s32    — [NSString length] returns NSUInteger (treated as s32 here)
+//   s64    — same shape, wider
+//   f64    — [NSNumber doubleValue] returns double
+//
+// Today (1.6a, xfail): the lowering rejects non-void returns with
+// a diagnostic. Snapshot pins the diagnostic.
+// Next (1.6b/c): emit_llvm builds a per-call-site LLVM function
+// type from (recv, sel, args, ret_ty), shares one declared
+// `@objc_msgSend` symbol, dispatches with the right ABI.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        // *void return: [NSObject class] → Class
+        cls := #objc_call(*void)(null, "class");
+        print("class non-null = {}\n", cls != null);
+    }
+    inline if OS != .macos {
+        print("skipped (not macos)\n");
+    }
+    0;
+}

tests/expected/ffi-objc-call-04-primitive-returns.exit

@@ -0,0 +1 @@
+1

tests/expected/ffi-objc-call-04-primitive-returns.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/ffi-objc-call-04-primitive-returns.sx: error: #objc_call: only `void` return + (recv, selector) is lowered today; non-void / arg-bearing arities land in later phase-1 steps