ffi M1.2 A.4b.i: __sx_state ivar registration

Class-pair init constructor now registers a single hidden ivar
on each sx-defined class:

  class_addIvar(cls, "__sx_state", 8, 3, "^v")

before objc_registerClassPair. After the class is registered,
the constructor calls class_getInstanceVariable to fetch the
runtime Ivar handle and stores it in a per-class global
'__<ClassName>_state_ivar : *void'. Trampolines (A.4b.ii) will
read this global to 'object_getIvar' the state struct pointer.

lower.zig declares the per-class global at scan time
(declareObjcDefinedStateIvarGlobal) so emit_llvm finds it by
name when populating. Encoding '^v' = void* (a generic pointer
— the runtime treats it as opaque storage). log2 alignment = 3
for 8-byte pointer alignment on 64-bit.

144-objc-class-ivar-registration.sx exercises the round-trip:
after main starts, class_getInstanceVariable(SxFoo, "__sx_state")
returns non-null. Runs against the real Obj-C runtime on macOS.

142's IR snapshot refreshed to include the new constructor body
(class_addIvar + class_getInstanceVariable + ivar-global store).

173 example tests pass (+1 from 144). zig build test green.

examples/144-objc-class-ivar-registration.sx

@@ -0,0 +1,51 @@
+// M1.2 A.4b.i — '__sx_state' ivar registration on sx-defined
+// '#objc_class'.
+//
+// Class-pair init now:
+//   1. allocs the class pair
+//   2. registers a single '__sx_state : *void' ivar
+//   3. finalises the class
+//   4. stores the runtime Ivar handle in a per-class global
+//      ('__<ClassName>_state_ivar') so IMP trampolines can later
+//      'object_getIvar' the state struct pointer.
+//
+// Round-trip below: after main starts, look up SxFoo, then ask
+// the runtime if SxFoo has an Ivar named '__sx_state'. Returns
+// non-null iff registration succeeded.
+//
+// IMP trampolines (A.4b.ii) and the '+alloc' / '-dealloc'
+// overrides (A.5 / A.6) come next.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+class_getInstanceVariable :: (cls: *void, name: [*]u8) -> *void #foreign objc;
+
+SxFoo :: #objc_class("SxFoo") {
+    counter: s32;
+
+    bump :: (self: *Self) {
+        self.counter += 1;
+    }
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        cls : Class = objc_getClass("SxFoo".ptr);
+        if cls == null {
+            print("FAIL: SxFoo not registered\n");
+            return 1;
+        }
+        iv := class_getInstanceVariable(cls, "__sx_state".ptr);
+        if iv == null {
+            print("FAIL: __sx_state ivar missing\n");
+            return 1;
+        }
+        print("ivar: __sx_state\n");
+    }
+    inline if OS != .macos {
+        print("ivar: __sx_state\n");
+    }
+    0;
+}