[flags] add enableEffectEventMutationPhase (#35548)

Small optimization for useEffectEvent. Not sure we even need a flag for
it, but it will be a nice killswitch.

As an added benefit, it fixes a bug when `enableViewTransition` is on,
where we were not updating the useEffectEvent callback when a tree went
from hidden to visible.

DiffTrain build for [3aaab92](3aaab92)

Author: rickhanlonii

compiled-rn/VERSION_NATIVE_FB

@@ -1 +1 @@
-19.3.0-native-fb-c137dd6f-20260204
+19.3.0-native-fb-3aaab92a-20260204

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react/react-dom/cjs/ReactDOM-dev.js

@@ -7,7 +7,7 @@
  * @noflow
  * @nolint
  * @preventMunge
- * @generated SignedSource<<c281d83b4dee94eb2fe9dd41b2915078>>
+ * @generated SignedSource<<d46ea7044f940d678465b87f5ab226ba>>
  */
 
 "use strict";
@@ -410,5 +410,5 @@ __DEV__ &&
     exports.useFormStatus = function () {
       return resolveDispatcher().useHostTransitionStatus();
     };
-    exports.version = "19.3.0-native-fb-c137dd6f-20260204";
+    exports.version = "19.3.0-native-fb-3aaab92a-20260204";
   })();

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react/react-dom/cjs/ReactDOM-prod.js

@@ -7,7 +7,7 @@
  * @noflow
  * @nolint
  * @preventMunge
- * @generated SignedSource<<c01afad83ef64def00c95d8a29367a13>>
+ * @generated SignedSource<<9288d76792bc59fdc0bc26343d568a8b>>
  */
 
 "use strict";
@@ -209,4 +209,4 @@ exports.useFormState = function (action, initialState, permalink) {
 exports.useFormStatus = function () {
   return ReactSharedInternals.H.useHostTransitionStatus();
 };
-exports.version = "19.3.0-native-fb-c137dd6f-20260204";
+exports.version = "19.3.0-native-fb-3aaab92a-20260204";

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react/react-dom/cjs/ReactDOM-profiling.js

@@ -7,7 +7,7 @@
  * @noflow
  * @nolint
  * @preventMunge
- * @generated SignedSource<<c01afad83ef64def00c95d8a29367a13>>
+ * @generated SignedSource<<9288d76792bc59fdc0bc26343d568a8b>>
  */
 
 "use strict";
@@ -209,4 +209,4 @@ exports.useFormState = function (action, initialState, permalink) {
 exports.useFormStatus = function () {
   return ReactSharedInternals.H.useHostTransitionStatus();
 };
-exports.version = "19.3.0-native-fb-c137dd6f-20260204";
+exports.version = "19.3.0-native-fb-3aaab92a-20260204";