ffi M1.3: obj.class accessor on Obj-C-class pointers

Adds a special case to lowerFieldAccess: when the field is
literally 'class' and the receiver is a pointer to an Obj-C
(or Obj-C protocol) foreign-class struct, emit
'object_getClass(obj)' instead of falling through to struct GEP.

Returns 'Class' (the M1.1 first-pass alias for *void;
parameterized Class(T) covariance is deferred to M1.1.b).

  f := SxFoo.alloc();
  cls := f.class;                       // → object_getClass(f)
  cls == objc_getClass("SxFoo".ptr);   // ok

New helper isObjcClassPointer(ty) detects 'ptr -> struct in
foreign_class_map under .objc_class / .objc_protocol'. The
check fires BEFORE the auto-deref so the runtime call sees the
opaque Obj-C pointer rather than the load'd struct stub.

148-objc-self-class-accessor.sx exercises both shapes end-to-end
against the macOS runtime: sx-defined class (SxFoo) and foreign
class (NSObject). Round-trips against objc_getClass(name).

178 example tests pass. zig build test green.

This effectively closes Month 1 — M1.0, M1.1 (first pass), M1.2,
M1.3 all done. Remaining: M1.1.b (Class(T) covariance +
instancetype), then Month 2 (declarative sugar).

examples/148-objc-self-class-accessor.sx

@@ -0,0 +1,48 @@
+// M1.3 — `obj.class` accessor on Obj-C pointers.
+//
+// Any Obj-C-class pointer (foreign or sx-defined) can be probed
+// for its runtime class object via `obj.class`. Lowers to
+// `object_getClass(obj)`. Returns `Class` (alias for *void —
+// parameterized `Class(T)` covariance is M1.1.b).
+//
+// Verifies both shapes:
+//   1. (*SxFoo).class — sx-defined class. Returns the SxFoo Class.
+//   2. (*NSObject).class — foreign class via stdlib. Returns NSObject's
+//      Class.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+NSObjectFwd :: #foreign #objc_class("NSObject") {
+    alloc :: () -> *NSObjectFwd;
+    init  :: (self: *NSObjectFwd) -> *NSObjectFwd;
+}
+
+SxFoo :: #objc_class("SxFoo") {
+    counter: s32;
+    alloc :: () -> *SxFoo;
+    bump :: (self: *Self) { self.counter += 1; }
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        // sx-defined class round-trip.
+        f := SxFoo.alloc();
+        cls_f : Class = f.class;
+        expected_f : Class = objc_getClass("SxFoo".ptr);
+        if cls_f != expected_f { print("FAIL: SxFoo.class mismatch\n"); return 1; }
+
+        // foreign class round-trip.
+        nso := NSObjectFwd.alloc().init();
+        cls_n : Class = nso.class;
+        expected_n : Class = objc_getClass("NSObject".ptr);
+        if cls_n != expected_n { print("FAIL: NSObject.class mismatch\n"); return 1; }
+
+        print("class accessor: ok\n");
+    }
+    inline if OS != .macos {
+        print("class accessor: ok\n");
+    }
+    0;
+}