ffi M3.2: SxSceneDelegate migrated + #implements protocol conformance

Migrates SxSceneDelegate from the hand-rolled
objc_allocateClassPair + class_addMethod + class_addProtocol
sequence to the declarative form:

  SxSceneDelegate :: #objc_class("SxSceneDelegate") {
      #extends UIResponder;
      #implements UISceneDelegate;
      #implements UIWindowSceneDelegate;

      scene_willConnectToSession_options :: (self, scene, session, options) { ... }
      window    :: (self) -> *void { ... }
      setWindow :: (self, w) { ... }
  }

emit_llvm now honors '#implements' in the class-pair init
constructor — for each #implements ProtocolAlias on the cache
entry's AST, emit before objc_registerClassPair:

  proto = objc_getProtocol("ProtocolName")
  class_addProtocol(cls, proto)

iOS checks 'class_conformsToProtocol' when instantiating scene
delegates; without the conformance the runtime silently rejects
the class and a default scene with no delegate gets created
instead. The protocol-getter returns null on dead-strip /
runtime mismatch (rare but possible) — the runtime treats
class_addProtocol(cls, null) as a no-op, so no explicit null
check needed.

Method bodies forward to the existing legacy free IMP functions
(uikit_scene_will_connect, uikit_window_getter,
uikit_window_setter) so we don't have to inline the scene-
connect setup logic (~80 lines).

uikit_register_classes is now tiny — just the two remaining
view-class helpers (M3.3 SxGLView + M3.4 SxMetalView). M3.5
deletes the function entirely once those land.

Chess on iOS-sim: board renders, scene delegate fires, touch
events route correctly. 183 example tests + zig build test
green.

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

@@ -878,3 +878,7 @@ entry:
 }
 
 declare ptr @object_getClass(ptr)
+
+declare ptr @objc_getProtocol(ptr)
+
+declare i8 @class_addProtocol(ptr, ptr)