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).