ffi M4.0a: prepend __sx_allocator to sx-defined-class state struct

State struct for an sx-defined `#objc_class` now leads with an
Allocator field at index 0 — captured at +alloc time, read by
-dealloc to free the state through the same allocator. User fields
shift to index 1+; the existing by-name lookups in
emitObjcDefinedClassPropertyImps + lookupObjcDefinedStateFieldOnPointer
naturally resolve them at the new indices.

This step is the layout change only; the +alloc IMP still mallocs
(M4.0b will rewrite it to thread context.allocator through), and
-dealloc still uses free() (M4.0c). The field is allocated but
uninitialised; nobody reads it yet.

Storage type comes from `Context.fields[0].ty` via the new
`objcStateAllocatorType` helper — same Allocator value-shape the
implicit context machinery has used all along. If Context isn't
registered (early-init paths), the helper falls back to omitting
the field rather than synthesising a half-broken layout.

IR snapshot for 142-objc-class-method-lowering updated to reflect
the new struct shape and the +24-byte state allocation. Chess on
iOS-sim green; 184/184 example tests pass.

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

@@ -740,7 +740,7 @@ entry:
   %load = load ptr, ptr %alloca, align 8
   %loadN = load ptr, ptr @__SxFoo_state_ivar, align 8
   %call = call ptr @object_getIvar(ptr %load, ptr %loadN)
-  %gep = getelementptr inbounds { i32 }, ptr %call, i32 0, i32 0
+  %gep = getelementptr inbounds { { ptr, ptr, ptr }, i32 }, ptr %call, i32 0, i32 1
   %loadN = load i32, ptr %gep, align 4
   %add = add i32 %loadN, 1
   store i32 %add, ptr %gep, align 4
@@ -800,8 +800,8 @@ declare ptr @object_getIvar(ptr, ptr) #0
 define ptr @__SxFoo_alloc_imp(ptr %0, ptr %1) #0 {
 entry:
   %call = call ptr @class_createInstance(ptr %0, i64 0)
-  %callN = call ptr @malloc(i64 4)
-  %callN = call ptr @memset(ptr %callN, i32 0, i64 4)
+  %callN = call ptr @malloc(i64 32)
+  %callN = call ptr @memset(ptr %callN, i32 0, i64 32)
   %load = load ptr, ptr @__SxFoo_state_ivar, align 8
   call void @object_setIvar(ptr %call, ptr %load, ptr %callN)
   ret ptr %call