ffi M1.2 A.4b.ii: emit C-ABI IMP trampolines (dead code pending class_addMethod)

For each bodied instance method on a sx-defined #objc_class,
emit a C-callconv trampoline function '__<Cls>_<method>_imp':

  void __SxFoo_bump_imp(ptr obj, ptr _cmd, ...user_args) {
      ivar  = load @__SxFoo_state_ivar
      state = object_getIvar(obj, ivar)
      call @SxFoo.bump(__sx_default_context, state, ...user_args)
      ret
  }

The trampoline bridges the Obj-C runtime's IMP calling convention
('id self, SEL _cmd, ...args' as C ABI) to the sx body's
default-callconv shape ('__sx_ctx ptr, state ptr, ...user_args').
Implicit context comes from '&__sx_default_context'; the body
keeps its sx-side personality intact and can use 'self.field'
through the substituted state-struct pointer (M1.2 A.2b + A.3).

New helpers in lower.zig:
- 'getObjcObjectGetIvarFid' lazily declares object_getIvar.
- 'emitObjcDefinedClassImps' + 'emitObjcDefinedClassImp' walk the
  cache and synthesise each trampoline.
- 'lookupGlobalIdByName' for finding the per-class ivar handle
  global. Linear scan — same N-is-small rationale as the other
  Obj-C caches.

Dead code at this commit: the trampolines exist in the module
but no class_addMethod call registers them with the runtime.
'objc_msgSend(obj, sel_bump)' would still fall through to the
parent class (NSObject 'doesNotRecognizeSelector:') today.
A.4b.iii wires up class_addMethod in emit_llvm's class-pair-init
constructor — that's when the trampolines come alive.

142's IR snapshot refreshed to show the trampoline.

173 example tests pass. zig build test green.

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

@@ -784,6 +784,18 @@ entry:
   ret { ptr, i64 } %call
 }
 
+; Function Attrs: nounwind
+define void @__SxFoo_bump_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 @SxFoo.bump(ptr @__sx_default_context, ptr %call)
+  ret void
+}
+
+; Function Attrs: nounwind
+declare ptr @object_getIvar(ptr, ptr) #0
+
 declare i64 @write(i32, ptr, i64)
 
 declare ptr @objc_getClass(ptr)