ffi M1.2 A.6: synthesized -dealloc IMP + [super dealloc] chain

For every sx-defined #objc_class, emit a C-callconv -dealloc IMP
that runs at refcount-zero. Frees the sx state struct, nils the
ivar, then chains to [super dealloc] so NSObject's runtime
cleanup (object_dispose, associated-object teardown, KVO, etc.)
runs as usual.

  -dealloc IMP (self: id, _cmd: SEL) -> void
      state = object_getIvar(self, load @__<Cls>_state_ivar)
      free(state)                              // free(NULL) is safe
      object_setIvar(self, ivar, NULL)
      sup = alloca { receiver: *void, super_class: *void }
      sup.receiver    = self
      sup.super_class = load @__<Cls>_class
      sel_dealloc = sel_registerName("dealloc")
      objc_msgSendSuper2(&sup, sel_dealloc)
      return

Two new per-class globals:
- '__<Cls>_class' : *void — populated by emit_llvm's
  class-pair init constructor with the freshly-allocated Class
  pointer (after objc_registerClassPair).
- The existing '__<Cls>_state_ivar' is also consulted to find
  the state struct.

The -dealloc IMP is registered on the class itself (instance
method) via class_addMethod with encoding 'v@:'. emit_llvm
ALSO stores cls_val into '__<Cls>_class' so the trampoline
can build the objc_super struct.

internStringConstantGlobal helper added to lower.zig — interns
C strings as [N:0]u8 globals with byte-level aggregate inits.
Used here for the 'dealloc' selector string.

147-objc-class-dealloc-roundtrip.sx verifies end-to-end on
macOS: alloc + release fires the IMP, and a second alloc/release
cycle proves runtime state isn't corrupted. class_getMethod-
Implementation confirms the IMP is registered.

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

Still gated: sx-side 'obj.method()' calls bail at lower.zig:4407
with the existing diagnostic. A.7 opens the gate — last sub-step
of M1.2.

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

@@ -3,7 +3,9 @@
 @ARCH = internal global i64 0
 @POINTER_SIZE = internal global i64 8
 @__SxFoo_state_ivar = internal global ptr null
+@__SxFoo_class = internal global ptr null
 @__sx_default_context = internal global { { ptr, ptr, ptr }, ptr } { { ptr, ptr, ptr } { ptr null, ptr @__thunk_CAllocator_Allocator_alloc, ptr @__thunk_CAllocator_Allocator_dealloc }, ptr null }
+@__sx_objc_cstr_dealloc = internal global [8 x i8] c"dealloc\00"
 @str = private unnamed_addr constant [2 x i8] c"0\00", align 1
 @str.1 = private unnamed_addr constant [15 x i8] c"result := \22\22; \00", align 1
 @str.2 = private unnamed_addr constant [37 x i8] c"result = concat(result, substr(fmt, \00", align 1
@@ -29,8 +31,10 @@
 @OBJC_CLASS_NAME_.19 = private unnamed_addr constant [6 x i8] c"SxFoo\00"
 @OBJC_METH_VAR_NAME_ = private unnamed_addr constant [5 x i8] c"bump\00"
 @OBJC_METH_VAR_TYPE_ = private unnamed_addr constant [4 x i8] c"v@:\00"
-@OBJC_METH_VAR_NAME_.20 = private unnamed_addr constant [6 x i8] c"alloc\00"
-@OBJC_METH_VAR_TYPE_.21 = private unnamed_addr constant [4 x i8] c"@@:\00"
+@OBJC_METH_VAR_NAME_.20 = private unnamed_addr constant [8 x i8] c"dealloc\00"
+@OBJC_METH_VAR_TYPE_.21 = private unnamed_addr constant [4 x i8] c"v@:\00"
+@OBJC_METH_VAR_NAME_.22 = private unnamed_addr constant [6 x i8] c"alloc\00"
+@OBJC_METH_VAR_TYPE_.23 = private unnamed_addr constant [4 x i8] c"@@:\00"
 @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @__sx_objc_defined_class_init, ptr null }]
 
 ; Function Attrs: nounwind
@@ -805,6 +809,33 @@ declare ptr @class_createInstance(ptr, i64) #0
 ; Function Attrs: nounwind
 declare void @object_setIvar(ptr, ptr, ptr) #0
 
+; Function Attrs: nounwind
+define void @__SxFoo_dealloc_imp(ptr %0, ptr %1) #0 {
+entry:
+  %load = load ptr, ptr @__SxFoo_state_ivar, align 8
+  %call = call ptr @object_getIvar(ptr %0, ptr %load)
+  call void @free(ptr %call)
+  call void @object_setIvar(ptr %0, ptr %load, ptr null)
+  %alloca = alloca { ptr, ptr }, align 8
+  %gep = getelementptr inbounds { ptr, ptr }, ptr %alloca, i32 0, i32 0
+  store ptr %0, ptr %gep, align 8
+  %loadN = load ptr, ptr @__SxFoo_class, align 8
+  %gepN = getelementptr inbounds { ptr, ptr }, ptr %alloca, i32 0, i32 1
+  store ptr %loadN, ptr %gepN, align 8
+  %callN = call ptr @sel_registerName(ptr @__sx_objc_cstr_dealloc)
+  call void @objc_msgSendSuper2(ptr %alloca, ptr %callN)
+  ret void
+}
+
+; Function Attrs: nounwind
+declare ptr @object_getIvar(ptr, ptr) #0
+
+; Function Attrs: nounwind
+declare ptr @sel_registerName(ptr) #0
+
+; Function Attrs: nounwind
+declare void @objc_msgSendSuper2(ptr, ptr) #0
+
 ; Function Attrs: nounwind
 define void @__SxFoo_bump_imp(ptr %0, ptr %1) #0 {
 entry:
@@ -814,9 +845,6 @@ entry:
   ret void
 }
 
-; Function Attrs: nounwind
-declare ptr @object_getIvar(ptr, ptr) #0
-
 declare i64 @write(i32, ptr, i64)
 
 declare ptr @objc_getClass(ptr)
@@ -825,8 +853,6 @@ declare ptr @objc_allocateClassPair(ptr, ptr, i64)
 
 declare i8 @class_addIvar(ptr, ptr, i64, i8, ptr)
 
-declare ptr @sel_registerName(ptr)
-
 declare i8 @class_addMethod(ptr, ptr, ptr, ptr)
 
 declare void @objc_registerClassPair(ptr)
@@ -841,9 +867,12 @@ entry:
   %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_.20)
-  %2 = call i8 @class_addMethod(ptr %metacls, ptr %sel_alloc, ptr @__SxFoo_alloc_imp, ptr @OBJC_METH_VAR_TYPE_.21)
+  %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_)
   store ptr %iv, ptr @__SxFoo_state_ivar, align 8
   ret void

tests/expected/147-objc-class-dealloc-roundtrip.exit

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

tests/expected/147-objc-class-dealloc-roundtrip.txt

@@ -0,0 +1 @@
+dealloc: ok