platform: UIKit backend renders GLES3 via CAEAGLLayer + CADisplayLink

End-to-end on iOS sim: UIKitPlatform boots an SxAppDelegate, installs
an SxGLView (UIView subclass overriding +layerClass to return
CAEAGLLayer) as the root view controller's view, sets the drawable
properties (EAGLColorFormatRGBA8, non-retained backing — looked up by
dlsym so pointer-identity-checked constants match), creates an
EAGLContext (GLES3), and registers a CADisplayLink that invokes the
user's frame closure on every vsync. end_frame presents the
renderbuffer via [EAGLContext presentRenderbuffer:].

The renderbuffer is allocated lazily in -[SxGLView layoutSubviews] once
the layer has its real on-screen bounds — allocating earlier (e.g. in
didFinishLaunching) failed with INCOMPLETE_ATTACHMENT because the
SxGLView's frame was still zero at that point. Setting the SxGLView
as the VC's `view` (via setView:) lets the standard VC layout pipeline
size it to the window without us having to read CGRect struct returns
from objc_msgSend.

EAGL drawableProperties dict keys/values are dlsym'd from OpenGLES —
the framework checks them by pointer identity, so synthesized NSString
literals with the same contents don't work.

examples/66-uikit-platform.sx — runnable smoke test that cycles the
screen color (red → green → blue every 30 frames) so you can confirm
the display-link tick and present pipeline.

modules/opengl.sx gains glGenFramebuffers, glGenRenderbuffers,
glBindFramebuffer, glBindRenderbuffer, glFramebufferRenderbuffer,
glGetRenderbufferParameteriv, glCheckFramebufferStatus — needed for
the iOS GLES FBO-to-renderbuffer setup. They're wired into load_gl
so SDL and the iOS dlsym loader both pick them up.

Compiles cleanly on macOS / WASM / iOS-sim. Non-iOS targets never
reference the unresolved UIKit/QuartzCore/OpenGLES symbols because
every Obj-C touch lives inside `inline if OS == .ios`.

Game's iOS path still goes through SDL3 for now. Touch events + game
wire-up + keyboard observer = next steps.

examples/66-uikit-platform.sx

@@ -0,0 +1,39 @@
+// UIKitPlatform end-to-end smoke: boots the AppDelegate, installs an
+// SxGLView with a CAEAGLLayer + GLES3 context + CADisplayLink, and on
+// every vsync clears the screen to a cycling color.
+//
+// Build + run:
+//   sx build --target ios-sim examples/66-uikit-platform.sx \
+//     -o /tmp/SxUIKitBoot --bundle /tmp/SxUIKitBoot.app \
+//     --bundle-id co.swipelab.sxuikit -F ~/Library/Frameworks
+//   xcrun simctl install booted /tmp/SxUIKitBoot.app
+//   xcrun simctl launch --console booted co.swipelab.sxuikit
+//   xcrun simctl io booted screenshot /tmp/screen.png
+
+#import "modules/std.sx";
+#import "modules/std/uikit.sx";
+#framework "OpenGLES";
+#framework "QuartzCore";
+#import "modules/opengl.sx";
+#import "modules/platform/uikit.sx";
+
+g_frame_counter : s64 = 0;
+
+cycle_frame :: () {
+    fc := g_uikit_plat.begin_frame();
+    g_frame_counter += 1;
+    phase := g_frame_counter / 30;
+    r : f32 = if (phase % 3) == 0 then 0.8 else 0.1;
+    g : f32 = if (phase % 3) == 1 then 0.8 else 0.1;
+    b : f32 = if (phase % 3) == 2 then 0.8 else 0.1;
+    glViewport(0, 0, fc.pixel_w, fc.pixel_h);
+    glClearColor(r, g, b, 1.0);
+    glClear(GL_COLOR_BUFFER_BIT);
+    g_uikit_plat.end_frame();
+}
+
+main :: () -> void {
+    plat : *UIKitPlatform = xx malloc(size_of(UIKitPlatform));
+    plat.init("SxUIKitPlatform", 0, 0);
+    plat.run_frame_loop(closure(cycle_frame));
+}