fix: keep readonly on instance Event phase declarations for Flow

The instance-level declarations need `readonly` to match their literal
types (0, 1, 2, 3). Without readonly, Flow treats them as invariant and
rejects the Object.defineProperty value assignments as incompatible-type.
The readonly annotation is type-level only (stripped at runtime); the
actual runtime fix is writable+configurable on Object.defineProperty.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: IsaacIsrael

packages/react-native/src/private/webapis/dom/events/Event.js

@@ -50,10 +50,10 @@ export default class Event {
   static readonly AT_TARGET: 2;
   static readonly BUBBLING_PHASE: 3;
 
-  NONE: 0;
-  CAPTURING_PHASE: 1;
-  AT_TARGET: 2;
-  BUBBLING_PHASE: 3;
+  readonly NONE: 0;
+  readonly CAPTURING_PHASE: 1;
+  readonly AT_TARGET: 2;
+  readonly BUBBLING_PHASE: 3;
 
   _bubbles: boolean;
   _cancelable: boolean;