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).

src/ir/lower.zig

@@ -3576,6 +3576,22 @@ pub const Lowering = struct {
             }
         }
 
+        // M1.3 — `obj.class` on any Obj-C-class pointer lowers to
+        // `object_getClass(obj)`. Sugar; the receiver is opaque so
+        // we don't auto-deref. Returns `Class` (alias for *void;
+        // typed Class(T) parameterization is M1.1.b).
+        if (std.mem.eql(u8, fa.field, "class")) {
+            const expr_ty = self.inferExprType(fa.object);
+            if (self.isObjcClassPointer(expr_ty)) {
+                const obj_ref = self.lowerExpr(fa.object);
+                const ptr_void = self.module.types.ptrTo(.void);
+                const get_class_fid = self.ensureCRuntimeDecl("object_getClass", &.{ptr_void}, ptr_void);
+                const args = self.alloc.alloc(Ref, 1) catch unreachable;
+                args[0] = obj_ref;
+                return self.builder.emit(.{ .call = .{ .callee = get_class_fid, .args = args } }, ptr_void);
+            }
+        }
+
         var obj = self.lowerExpr(fa.object);
         var obj_ty = self.inferExprType(fa.object);
 
@@ -11530,6 +11546,21 @@ pub const Lowering = struct {
         self.emitObjcDefinedClassImps();
     }
 
+    /// True if `ty` is a pointer to a struct whose name is registered
+    /// in `foreign_class_map` under an Obj-C runtime. Used by the
+    /// `obj.class` accessor (M1.3) to decide whether to lower the
+    /// field access as a struct GEP or as `object_getClass(obj)`.
+    fn isObjcClassPointer(self: *Lowering, ty: TypeId) bool {
+        if (ty.isBuiltin()) return false;
+        const ptr_info = self.module.types.get(ty);
+        if (ptr_info != .pointer) return false;
+        const pointee_info = self.module.types.get(ptr_info.pointer.pointee);
+        if (pointee_info != .@"struct") return false;
+        const struct_name = self.module.types.getString(pointee_info.@"struct".name);
+        const fcd = self.foreign_class_map.get(struct_name) orelse return false;
+        return fcd.runtime == .objc_class or fcd.runtime == .objc_protocol;
+    }
+
     /// Get a FuncId for an external C-callconv function. If a function
     /// with this exported name already exists in the module (e.g.
     /// declared by stdlib `#foreign` decl), return it; otherwise