ffi M2.1(b): class methods on sx-defined #objc_class

Bodied methods without a '*Self' first param (parser marks
is_static=true) are now registered as Obj-C CLASS methods on
the metaclass.

Each such method gets:
- A synthesized FnDecl + body lowering through the existing
  M1.2 A.2 path.
- A C-ABI trampoline 'emitObjcDefinedClassStaticImp' — same
  shape as the instance trampoline but skips the __sx_state
  ivar read (no instance state) and passes only
  '__sx_default_context' (plus user args) to the sx body.
- An entry in ObjcDefinedMethodEntry with 'is_class=true'.

emit_llvm's class-pair init constructor now computes the
metaclass once up-front (via object_getClass(cls)) and shares
it between the +alloc IMP registration (M1.2 A.5) and the
M2.1(b) class-method registrations. The per-method registration
loop picks the target via 'method.is_class ? metaclass : cls'.

149-objc-class-method-static-imp.sx end-to-end on macOS:

  SxFoo :: #objc_class("SxFoo") {
      answer :: () -> s32 { return 42; }
  }

  // [SxFoo answer] via objc_msgSend → 42
  // class_getClassMethod(SxFoo, sel_answer) → non-null

Still TODO for M2.1: the (a) class-LEVEL constant form
'layerClass :: Class = CAEAGLLayer.class();' — needs parser
extension to recognize 'name :: Type = expr;' inside #objc_class
blocks, plus lazy-init-slot synthesis.

179 example tests pass (+1). zig build test green.

tests/expected/142-objc-class-method-lowering.ir

@@ -864,13 +864,13 @@ entry:
   %super_cls = call ptr @objc_getClass(ptr @OBJC_CLASS_NAME_)
   %cls = call ptr @objc_allocateClassPair(ptr %super_cls, ptr @OBJC_CLASS_NAME_.19, i64 0)
   %0 = call i8 @class_addIvar(ptr %cls, ptr @OBJC_IVAR_NAME_, i64 8, i8 3, ptr @OBJC_IVAR_TYPE_)
+  %metacls = call ptr @object_getClass(ptr %cls)
   %sel = call ptr @sel_registerName(ptr @OBJC_METH_VAR_NAME_)
   %1 = call i8 @class_addMethod(ptr %cls, ptr %sel, ptr @__SxFoo_bump_imp, ptr @OBJC_METH_VAR_TYPE_)
   call void @objc_registerClassPair(ptr %cls)
   store ptr %cls, ptr @__SxFoo_class, align 8
   %sel_dealloc = call ptr @sel_registerName(ptr @OBJC_METH_VAR_NAME_.20)
   %2 = call i8 @class_addMethod(ptr %cls, ptr %sel_dealloc, ptr @__SxFoo_dealloc_imp, ptr @OBJC_METH_VAR_TYPE_.21)
-  %metacls = call ptr @object_getClass(ptr %cls)
   %sel_alloc = call ptr @sel_registerName(ptr @OBJC_METH_VAR_NAME_.22)
   %3 = call i8 @class_addMethod(ptr %metacls, ptr %sel_alloc, ptr @__SxFoo_alloc_imp, ptr @OBJC_METH_VAR_TYPE_.23)
   %iv = call ptr @class_getInstanceVariable(ptr %cls, ptr @OBJC_IVAR_NAME_)

tests/expected/149-objc-class-method-static-imp.exit

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

tests/expected/149-objc-class-method-static-imp.txt

@@ -0,0 +1 @@
+class method: 42