ui: platform-neutral Keycode enum; map SDL keycodes once

KeyData.key was a raw u32 carrying SDL_Keycode values, so app code had to reinterpret it as SDL_Keycode (xx e.key) — a leaky, unchecked cross-platform cast only valid because the backend happened to be SDL. Add a neutral Keycode enum; translate_sdl_event maps SDL_Keycode to it via keycode_from_sdl. App code compares e.key == .escape with no platform type and no cast; a new backend maps its own native codes in one place.

library/modules/ui/events.sx

@@ -12,7 +12,24 @@ MouseButton :: enum {
 MouseButtonData :: struct { position: Point; button: MouseButton; }
 MouseMotionData :: struct { position: Point; delta: Point; }
 MouseWheelData  :: struct { position: Point; delta: Point; }
-KeyData         :: struct { key: u32; }
+
+// Platform-neutral keycode. Each backend translates its native codes to this
+// (see `keycode_from_sdl`), so app code never depends on a platform key enum.
+Keycode :: enum {
+    unknown;
+    escape;
+    enter;
+    space;
+    tab;
+    backspace;
+    delete;
+    left;
+    right;
+    up;
+    down;
+}
+
+KeyData         :: struct { key: Keycode; }
 ResizeData      :: struct { size: Size; }
 
 Event :: enum {
@@ -38,6 +55,26 @@ event_position :: (e: *Event) -> ?Point {
     null;
 }
 
+// Map a platform (SDL) keycode to the neutral `Keycode`. Only the keys the app
+// consumes are mapped; everything else is `.unknown`. Letters/digits arrive via
+// `text_input`, not here.
+// TODO: extend as more keys are needed.
+keycode_from_sdl :: (k: SDL_Keycode) -> Keycode {
+    if k == {
+        case .escape:     return .escape;
+        case .return_key: return .enter;
+        case .space:      return .space;
+        case .tab:        return .tab;
+        case .backspace:  return .backspace;
+        case .delete_key: return .delete;
+        case .left:       return .left;
+        case .right:      return .right;
+        case .up:         return .up;
+        case .down:       return .down;
+    }
+    .unknown;
+}
+
 // Translate SDL_Event → our Event type
 translate_sdl_event :: (sdl: *SDL_Event) -> Event {
     if sdl.* == {
@@ -45,10 +82,10 @@ translate_sdl_event :: (sdl: *SDL_Event) -> Event {
             return .quit;
         }
         case .key_down: (data) {
-            return .key_down(.{ key = xx data.key });
+            return .key_down(.{ key = keycode_from_sdl(data.key) });
         }
         case .key_up: (data) {
-            return .key_up(.{ key = xx data.key });
+            return .key_up(.{ key = keycode_from_sdl(data.key) });
         }
         case .mouse_motion: (data) {
             return .mouse_moved(.{