[Fizz] Track whether we're in a fallback on FormatContext (#33194)

Removes the `isFallback` flag on Tasks and tracks it on the
formatContext instead.

Less memory and avoids passing and tracking extra arguments to all the
pushStartInstance branches that doesn't need it.

We'll need to be able to track more Suspense related contexts on this
for View Transitions anyway.

DiffTrain build for [3f67d08](3f67d08)

Author: sebmarkbage

compiled/facebook-www/REVISION

@@ -1 +1 @@
-96eb84e493c4ff2c280990659057164c0f16bbb8
+3f67d0857efc3ab21b9d30851f5a8451471166ab

compiled/facebook-www/REVISION_TRANSFORMS

@@ -1 +1 @@
-96eb84e493c4ff2c280990659057164c0f16bbb8
+3f67d0857efc3ab21b9d30851f5a8451471166ab

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

@@ -1537,7 +1537,7 @@ __DEV__ &&
     exports.useTransition = function () {
       return resolveDispatcher().useTransition();
     };
-    exports.version = "19.2.0-www-classic-96eb84e4-20250514";
+    exports.version = "19.2.0-www-classic-3f67d085-20250515";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

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

@@ -1537,7 +1537,7 @@ __DEV__ &&
     exports.useTransition = function () {
       return resolveDispatcher().useTransition();
     };
-    exports.version = "19.2.0-www-modern-96eb84e4-20250514";
+    exports.version = "19.2.0-www-modern-3f67d085-20250515";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/React-prod.classic.js

@@ -635,4 +635,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.2.0-www-classic-96eb84e4-20250514";
+exports.version = "19.2.0-www-classic-3f67d085-20250515";

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

@@ -635,4 +635,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.2.0-www-modern-96eb84e4-20250514";
+exports.version = "19.2.0-www-modern-3f67d085-20250515";

compiled/facebook-www/React-profiling.classic.js

@@ -639,7 +639,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.2.0-www-classic-96eb84e4-20250514";
+exports.version = "19.2.0-www-classic-3f67d085-20250515";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

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

@@ -639,7 +639,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.2.0-www-modern-96eb84e4-20250514";
+exports.version = "19.2.0-www-modern-3f67d085-20250515";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/ReactART-dev.classic.js

@@ -19063,10 +19063,10 @@ __DEV__ &&
     (function () {
       var internals = {
         bundleType: 1,
-        version: "19.2.0-www-classic-96eb84e4-20250514",
+        version: "19.2.0-www-classic-3f67d085-20250515",
         rendererPackageName: "react-art",
         currentDispatcherRef: ReactSharedInternals,
-        reconcilerVersion: "19.2.0-www-classic-96eb84e4-20250514"
+        reconcilerVersion: "19.2.0-www-classic-3f67d085-20250515"
       };
       internals.overrideHookState = overrideHookState;
       internals.overrideHookStateDeletePath = overrideHookStateDeletePath;
@@ -19100,7 +19100,7 @@ __DEV__ &&
     exports.Shape = Shape;
     exports.Surface = Surface;
     exports.Text = Text;
-    exports.version = "19.2.0-www-classic-96eb84e4-20250514";
+    exports.version = "19.2.0-www-classic-3f67d085-20250515";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

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

@@ -18835,10 +18835,10 @@ __DEV__ &&
     (function () {
       var internals = {
         bundleType: 1,
-        version: "19.2.0-www-modern-96eb84e4-20250514",
+        version: "19.2.0-www-modern-3f67d085-20250515",
         rendererPackageName: "react-art",
         currentDispatcherRef: ReactSharedInternals,
-        reconcilerVersion: "19.2.0-www-modern-96eb84e4-20250514"
+        reconcilerVersion: "19.2.0-www-modern-3f67d085-20250515"
       };
       internals.overrideHookState = overrideHookState;
       internals.overrideHookStateDeletePath = overrideHookStateDeletePath;
@@ -18872,7 +18872,7 @@ __DEV__ &&
     exports.Shape = Shape;
     exports.Surface = Surface;
     exports.Text = Text;
-    exports.version = "19.2.0-www-modern-96eb84e4-20250514";
+    exports.version = "19.2.0-www-modern-3f67d085-20250515";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&