ffi 1.3: xfail end-to-end void-return #objc_call (no codegen yet)

100/100 regression tests pass (+ffi-objc-call-02-void-return xfail
snapshot).

The intrinsic with no `inline if false` guard reaches sema/codegen
and trips an "unresolved: 'unknown_expr'" — the FfiIntrinsicCall
AST node from Phase 1.1 has no lowering rules in lower.zig /
emit_llvm.zig yet.

nil receiver was chosen so the test doesn't need a real Obj-C
object graph: the runtime guarantees `[nil msg]` is a no-op with
zero result for void returns. macOS-gated via `inline if OS == .macos`
so the runner stays portable.

Next commit: emit_llvm.zig produces the per-call-site
  %sel = call ptr @sel_registerName(ptr "init.0")
  call void @objc_msgSend(ptr null, ptr %sel)
lowering. Snapshot flips to "ok". Selector interning (one shared
global per unique selector string) lands as a separate step (1.5).

examples/ffi-objc-call-02-void-return.sx

@@ -0,0 +1,25 @@
+// Phase 1 step 1.3 (PLAN-FFI.md): smallest end-to-end `#objc_call`.
+// Void return, nil receiver — Obj-C runtime guarantees that messages
+// to nil are no-ops with a zero result, so we don't need to set up
+// a real object graph to exercise the lowering surface.
+//
+// Today (step 1.3, test-add): codegen rejects the FfiIntrinsicCall
+// AST node. Snapshot pins the failure mode.
+// Next (step 1.3, make-green): emit_llvm.zig synthesizes
+//   %sel = call ptr @sel_registerName(ptr @"sel:init")
+//   call void @objc_msgSend(ptr null, ptr %sel)
+// per call site (no selector interning until step 1.5).
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        #objc_call(void)(null, "init");
+        print("ok\n");
+    }
+    inline if OS != .macos {
+        print("skipped (not macos)\n");
+    }
+    0;
+}

tests/expected/ffi-objc-call-02-void-return.exit

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

tests/expected/ffi-objc-call-02-void-return.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/ffi-objc-call-02-void-return.sx:18:9: error: unresolved: 'unknown_expr'