[DevTools] Give a distinct color to the root (facebook#34690)

Stacked on facebook#34654.

The root is special since it represents "Initial Paint" (or a
"Transition" when an Activity is selected). This gives it a different
color in the timeline as well as gives it an outline that's clickable.
Hovering the timeline now shows "Initial Paint" or "Suspense".

Also made the cursor a pointer to invite you to try to click things and
some rounded corners.

<img width="1219" height="420" alt="Screenshot 2025-10-02 at 1 26 38 PM"
src="https://github.com/user-attachments/assets/12451f93-8917-4f3b-8f01-930129e5fc13"
/>

<img width="1217" height="419" alt="Screenshot 2025-10-02 at 1 26 54 PM"
src="https://github.com/user-attachments/assets/02b5e94c-3fbe-488d-b0f2-225b73578608"
/>

<img width="1215" height="419" alt="Screenshot 2025-10-02 at 1 27 24 PM"
src="https://github.com/user-attachments/assets/c24e8861-e74a-4ccc-8643-ee9d04bef43c"
/>

<img width="1216" height="419" alt="Screenshot 2025-10-02 at 1 27 10 PM"
src="https://github.com/user-attachments/assets/d5cc2b62-fa64-41bf-b485-116b1cd67467"
/>

DiffTrain build for [2e68dc7](facebook@2e68dc7)

Author: pull[bot]

compiled-rn/VERSION_NATIVE_FB

@@ -1 +1 @@
-19.2.0-native-fb-ef889445-20250930
+19.3.0-native-fb-2e68dc76-20251002

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

@@ -7,7 +7,7 @@
  * @noflow
  * @nolint
  * @preventMunge
- * @generated SignedSource<<f9ccd5dad29ba39f13f47a184d09a8fe>>
+ * @generated SignedSource<<8b5b9e0ff704bdb2135f1897a3fd5fbe>>
  */
 
 "use strict";
@@ -404,5 +404,5 @@ __DEV__ &&
     exports.useFormStatus = function () {
       return resolveDispatcher().useHostTransitionStatus();
     };
-    exports.version = "19.2.0-native-fb-ef889445-20250930";
+    exports.version = "19.3.0-native-fb-2e68dc76-20251002";
   })();

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

@@ -7,7 +7,7 @@
  * @noflow
  * @nolint
  * @preventMunge
- * @generated SignedSource<<dda06da31d489376c47d0e051387241b>>
+ * @generated SignedSource<<b3a0db0d2e40b6411b40f265a4ae4f35>>
  */
 
 "use strict";
@@ -203,4 +203,4 @@ exports.useFormState = function (action, initialState, permalink) {
 exports.useFormStatus = function () {
   return ReactSharedInternals.H.useHostTransitionStatus();
 };
-exports.version = "19.2.0-native-fb-ef889445-20250930";
+exports.version = "19.3.0-native-fb-2e68dc76-20251002";

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

@@ -7,7 +7,7 @@
  * @noflow
  * @nolint
  * @preventMunge
- * @generated SignedSource<<dda06da31d489376c47d0e051387241b>>
+ * @generated SignedSource<<b3a0db0d2e40b6411b40f265a4ae4f35>>
  */
 
 "use strict";
@@ -203,4 +203,4 @@ exports.useFormState = function (action, initialState, permalink) {
 exports.useFormStatus = function () {
   return ReactSharedInternals.H.useHostTransitionStatus();
 };
-exports.version = "19.2.0-native-fb-ef889445-20250930";
+exports.version = "19.3.0-native-fb-2e68dc76-20251002";