platform: iOS safe-area insets + keyboard observer

UIKitPlatform now reads `[UIView safeAreaInsets]` (UIEdgeInsets = 32-byte
struct: top, left, bottom, right CGFloats) in begin_frame, and subscribes
to UIKeyboardWillChangeFrameNotification on NSNotificationCenter. The
chess game's build_ui pads its root by `g_safe_insets`, so the Dynamic
Island no longer overlaps the board on iPhone 17 Pro — all 8 ranks and
files are visible.

Struct returns >16 bytes (UIEdgeInsets, CGRect) go through the arm64
x8 indirect-result-pointer convention; expressing the return type on a
typed `objc_msgSend` fn-pointer cast generates the right call sequence.
Same pattern used to unwrap the keyboard's CGRect from NSValue
(UIKeyboardFrameEndUserInfoKey).

show_keyboard / hide_keyboard now drive a hidden UITextField subview as
the firstResponder source. resignFirstResponder dismisses; observer
fires with height=0 → safe_insets bottom collapses.

Deferred (next iteration): wrap the inset update in
[UIView animateWithDuration: animations:^{ ... }] to land in the same
CoreAnimation transaction as the keyboard. sx doesn't have block
syntax yet — we'd need a C shim that takes an fn-ptr and builds the
block. Today the inset snaps while the keyboard slides; the lag is
visible but the rest of the wiring is in place.

examples/66-uikit-platform.sx updated: each tap toggles the keyboard
+ advances the clear color (red→green→blue), so the observer can be
observed firing via the visible keyboard slide.

examples/66-uikit-platform.sx

@@ -1,7 +1,9 @@
 // UIKitPlatform end-to-end smoke: boots the AppDelegate, installs an
 // SxGLView with a CAEAGLLayer + GLES3 context + CADisplayLink, polls
 // UITouch events into ui.Event, and on every vsync clears the screen
-// to a color that advances on each tap.
+// to a color that advances on each tap. Each tap also toggles the
+// on-screen keyboard so safe_insets.bottom can be observed growing /
+// shrinking under it.
 //
 // Build + run:
 //   sx build --target ios-sim examples/66-uikit-platform.sx \
@@ -21,6 +23,7 @@
 #import "modules/platform/uikit.sx";
 
 g_color_index : s64 = 0;
+g_keyboard_up : bool = false;
 
 tap_frame :: () {
     fc := g_uikit_plat.begin_frame();
@@ -30,7 +33,16 @@ tap_frame :: () {
     while i < events.len {
         ev := events.ptr[i];
         if ev == {
-            case .mouse_down: { g_color_index += 1; }
+            case .mouse_down: {
+                g_color_index += 1;
+                if g_keyboard_up {
+                    g_uikit_plat.hide_keyboard();
+                    g_keyboard_up = false;
+                } else {
+                    g_uikit_plat.show_keyboard();
+                    g_keyboard_up = true;
+                }
+            }
         }
         i += 1;
     }