Move ReactDOMLegacy implementation into RootFB (#28656)

Only the FB entry point has legacy mode now so we can move the remaining
code in there.

Also enable disableLegacyMode in modern www builds since it doesn't
expose those entry points.

Now dependent on #28709.

---------

Co-authored-by: Josh Story <story@hey.com>

DiffTrain build for [8f55a6a](8f55a6a)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-5de8703646cdd3838cb1686f761b10c0692743aa
+8f55a6aa5739ed8ca80c3066fb54f4ea4cfe600a

compiled/facebook-www/React-dev.modern.js

@@ -24,7 +24,7 @@ if (__DEV__) {
     ) {
       __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStart(new Error());
     }
-    var ReactVersion = "19.0.0-www-modern-e4b79a21";
+    var ReactVersion = "19.0.0-www-modern-cf1ffacc";
 
     // ATTENTION
     // When adding new symbols to this file,
@@ -404,7 +404,7 @@ if (__DEV__) {
       enableRenderableContext = dynamicFeatureFlags.enableRenderableContext,
       enableRefAsProp = dynamicFeatureFlags.enableRefAsProp;
     // On WWW, true is used for a new modern build.
-    var disableLegacyMode = false;
+    var disableLegacyMode = true;
 
     /*
      * The `'' + value` pattern (used in perf-sensitive code) throws for Symbol
@@ -3272,19 +3272,12 @@ if (__DEV__) {
         // `act` calls can be nested.
         //
         // If we're already inside an `act` scope, reuse the existing queue.
-        var prevIsBatchingLegacy = ReactCurrentActQueue.isBatchingLegacy;
+        var prevIsBatchingLegacy = false;
         var prevActQueue = ReactCurrentActQueue.current;
         var prevActScopeDepth = actScopeDepth;
         actScopeDepth++;
         var queue = (ReactCurrentActQueue.current =
           prevActQueue !== null ? prevActQueue : []); // Used to reproduce behavior of `batchedUpdates` in legacy mode. Only
-        // set to `true` while the given callback is executed, not for updates
-        // triggered during an async event, because this is how the legacy
-        // implementation of `act` behaved.
-
-        {
-          ReactCurrentActQueue.isBatchingLegacy = true;
-        }
 
         var result; // This tracks whether the `act` call is awaited. In certain cases, not
         // awaiting it is a mistake, so we will detect that and warn.
@@ -3295,9 +3288,7 @@ if (__DEV__) {
           // Reset this to `false` right before entering the React work loop. The
           // only place we ever read this fields is just below, right after running
           // the callback. So we don't need to reset after the callback runs.
-          if (!disableLegacyMode) {
-            ReactCurrentActQueue.didScheduleLegacyUpdate = false;
-          }
+          if (!disableLegacyMode);
 
           result = callback();
           var didScheduleLegacyUpdate = !disableLegacyMode
@@ -3313,9 +3304,7 @@ if (__DEV__) {
           // that's how it worked before version 18. Yes, it's confusing! We should
           // delete legacy mode!!
 
-          if (!disableLegacyMode) {
-            ReactCurrentActQueue.isBatchingLegacy = prevIsBatchingLegacy;
-          }
+          if (!disableLegacyMode);
         } catch (error) {
           // `isBatchingLegacy` gets reset using the regular stack, not the async
           // one used to track `act` scopes. Why, you may be wondering? Because
@@ -3325,10 +3314,6 @@ if (__DEV__) {
         }
 
         if (ReactCurrentActQueue.thrownErrors.length > 0) {
-          {
-            ReactCurrentActQueue.isBatchingLegacy = prevIsBatchingLegacy;
-          }
-
           popActScope(prevActQueue, prevActScopeDepth);
           var thrownError = aggregateErrors(ReactCurrentActQueue.thrownErrors);
           ReactCurrentActQueue.thrownErrors.length = 0;