ffi 3.2 C3: migrate uikit.sx RunLoop + display-timing cluster

Third cluster: NSRunLoop and CADisplayLink move to declarative
`#objc_class` blocks.

Classes declared:

- NSRunLoop     → currentRunLoop (class)
- CADisplayLink → displayLinkWithTarget_selector (class),
                  addToRunLoop_forMode (instance),
                  targetTimestamp (instance), duration (instance)

The display-link instance is created with the new typed call shape:

    link := CADisplayLink.displayLinkWithTarget_selector(plat.gl_view, sel_tick);
    plat.display_link = xx link;  // keep the *void slot in the
                                   // platform struct for ABI parity
    runloop := NSRunLoop.currentRunLoop();
    link.addToRunLoop_forMode(runloop, mode_ns);

The `sxTick:` callback's `link: *void` param is cast to
`*CADisplayLink` at function entry so the body's `link.duration()` /
`link.targetTimestamp()` calls type-check.

Two now-redundant `objc_getClass(...)` lookups for CADisplayLink /
NSRunLoop are gone — the class slots come from the declarative
declarations via `__sx_objc_class_init`.

166/166 tests; chess builds clean on macOS / iOS / Android.

current/CHECKPOINT-FFI.md

@@ -560,9 +560,17 @@ declared as `#foreign #objc_class` blocks. The 4-keyword
 `addObserver:selector:name:object:` selector derives cleanly from
 the underscore-separated sx name (`addObserver_selector_name_object`).
 
+Phase 3.2 C3 landed: RunLoop + display-timing cluster. NSRunLoop
+(`currentRunLoop`) and CADisplayLink
+(`displayLinkWithTarget_selector`, `addToRunLoop_forMode`,
+`targetTimestamp`, `duration`) declared as `#foreign #objc_class`
+blocks. The `link` parameter on the `sxTick:` callback is now cast
+to `*CADisplayLink` at function entry so subsequent method calls
+type-check.
+
 Open work:
-- **Phase 3 step 3.2 — C3..C5** — uikit.sx migration, one cluster
-  per commit, chess regression after each.
+- **Phase 3 step 3.2 — C4..C5** — uikit.sx migration, two clusters
+  remaining (UIKit chrome, view tree + GL).
   test for the default-mangling table. Escape hatch for selectors
   that don't fit the underscore-split rule (e.g. `tableView_
   numberOfRowsInSection_` with an asymmetric keyword count).

library/modules/platform/uikit.sx

@@ -100,6 +100,19 @@ NSNotificationCenter :: #foreign #objc_class("NSNotificationCenter") {
     addObserver_selector_name_object :: (self: *Self, observer: *void, sel: *void, name: *void, obj: *void);
 }
 
+// ── RunLoop + display timing (Phase 3.2 C3) ────────────────────────────
+
+NSRunLoop :: #foreign #objc_class("NSRunLoop") {
+    currentRunLoop :: () -> *NSRunLoop;
+}
+
+CADisplayLink :: #foreign #objc_class("CADisplayLink") {
+    displayLinkWithTarget_selector :: (target: *void, sel: *void) -> *CADisplayLink;
+    addToRunLoop_forMode :: (self: *Self, runloop: *NSRunLoop, mode: *void);
+    targetTimestamp :: (self: *Self) -> f64;
+    duration :: (self: *Self) -> f64;
+}
+
 // GLenum constants for renderbuffer/framebuffer setup that aren't in opengl.sx's
 // loader path (they live on the framework's symbol table directly).
 GL_RENDERBUFFER          :u32: 0x8D41;
@@ -534,8 +547,9 @@ uikit_scene_will_connect_ios :: (delegate: *void, scene: *void) {
     UIViewController := objc_getClass("UIViewController".ptr);
     SxGLView         := objc_getClass("SxGLView".ptr);
     SxMetalView      := objc_getClass("SxMetalView".ptr);
-    CADisplayLink    := objc_getClass("CADisplayLink".ptr);
-    NSRunLoop        := objc_getClass("NSRunLoop".ptr);
+    // CADisplayLink and NSRunLoop class objects come from the
+    // declarative `#objc_class` slots populated by emit_llvm's
+    // __sx_objc_class_init constructor — no local objc_getClass needed.
 
     win_raw := #objc_call(*void)(UIWindow, "alloc");
     plat.window = #objc_call(*void)(win_raw, "initWithWindowScene:", scene);
@@ -619,10 +633,11 @@ uikit_scene_will_connect_ios :: (delegate: *void, scene: *void) {
     // a SEL value (not a dispatch selector), so it still goes through
     // sel_registerName.
     sel_tick := sel_registerName("sxTick:".ptr);
-    plat.display_link = #objc_call(*void)(CADisplayLink, "displayLinkWithTarget:selector:", plat.gl_view, sel_tick);
-    runloop := #objc_call(*void)(NSRunLoop, "currentRunLoop");
-    mode := ns_string("kCFRunLoopDefaultMode".ptr);
-    #objc_call(void)(plat.display_link, "addToRunLoop:forMode:", runloop, mode);
+    link := CADisplayLink.displayLinkWithTarget_selector(plat.gl_view, sel_tick);
+    plat.display_link = xx link;
+    runloop := NSRunLoop.currentRunLoop();
+    mode_ns := ns_string("kCFRunLoopDefaultMode".ptr);
+    link.addToRunLoop_forMode(runloop, mode_ns);
 
     NSLog(ns_string("[sx] UIKitPlatform booted\n".ptr));
 }
@@ -682,7 +697,8 @@ uikit_gl_view_layer_class :: (cls: *void, _cmd: *void) -> *void callconv(.c) {
     objc_getClass("CAEAGLLayer".ptr);
 }
 
-uikit_gl_view_tick :: (self: *void, _cmd: *void, link: *void) callconv(.c) {
+uikit_gl_view_tick :: (self: *void, _cmd: *void, link_raw: *void) callconv(.c) {
+    link : *CADisplayLink = xx link_raw;
     if g_uikit_plat == null { return; }
     plat := g_uikit_plat;
 
@@ -696,7 +712,7 @@ uikit_gl_view_tick :: (self: *void, _cmd: *void, link: *void) callconv(.c) {
     // and keyboardLayoutGuide — none eliminated the lag without
     // cascade-breaking the GL view's frame.
     if plat.kb_animating {
-        target_ts := #objc_call(f64)(link, "targetTimestamp");
+        target_ts := link.targetTimestamp();
         elapsed := target_ts - plat.kb_anim_start;
         // Negative elapsed can happen if the just-fired willChangeFrame
         // set kb_anim_start to a wall time AFTER the tick already
@@ -723,11 +739,11 @@ uikit_gl_view_tick :: (self: *void, _cmd: *void, link: *void) callconv(.c) {
     if !plat.has_frame_closure { return; }
     if !plat.gl_initialized { return; }
 
-    dur_d := #objc_call(f64)(link, "duration");
+    dur_d := link.duration();
     plat.delta_time = xx dur_d;
     // Stash the targetTimestamp so begin_frame can hand it down to the
     // game in FrameContext for Metal presentDrawable:atTime:.
-    plat.last_target_ts = #objc_call(f64)(link, "targetTimestamp");
+    plat.last_target_ts = link.targetTimestamp();
 
     fn := plat.frame_closure;
     fn();