uikit: type UIKitPlatform fields properly + handle optional in Obj-C encoding

The UIKitPlatform struct had a string of '*void = null; // UIWindow*'
fields — the type lived in a comment, every callsite had to 'xx'-cast
back to the real type. Migrated to the real foreign-class pointer
types now that M3 declared all the relevant '#objc_class' aliases:

  window:       ?*UIWindow
  root_vc:      ?*UIViewController
  gl_view:      ?*UIView         (SxGLView OR SxMetalView — both extend UIView)
  gl_layer:     ?*CALayer        (CAEAGLLayer OR CAMetalLayer)
  gl_ctx:       ?*EAGLContext
  display_link: ?*CADisplayLink

Each field is wrapped in '?' since the platform may not have set
it yet (gl_ctx is null in metal mode, display_link is null before
the first frame, etc.).

SxSceneDelegate's window getter/setter now take/return '?*UIWindow'
instead of '*void' so calling code doesn't need an xx-cast.

Required fix in objcTypeEncodingFromSignature: '?T' (optional) was
bailing with 'type kind not yet supported'. Apple's runtime treats
nullability as 'pointer may be null' — the wire encoding is the
same as T. Recursive unwrap handles ?*UIView → '@', ?*CADisplayLink
→ '@', etc.

Chess on iOS-sim: board renders, full pipeline intact. 183 tests
+ zig build test green.

src/ir/lower.zig

@@ -4713,6 +4713,14 @@ pub const Lowering = struct {
                     try out.appendSlice(self.alloc, "^v");
                 }
             },
+            .optional => |o| {
+                // sx's `?T` is a nullable T. At the Obj-C ABI boundary
+                // nullability is just "this pointer may be null" — the
+                // wire-level encoding is the same as T. Unwrap and
+                // recurse. (Same goes for `?*UIView` etc. — the
+                // underlying pointer kind drives the encoding char.)
+                return self.appendObjcEncoding(out, o.child, span);
+            },
             else => return self.bailObjcEncoding(span, "type kind not yet supported by Obj-C encoding", @intFromEnum(std.meta.activeTag(info))),
         }
     }