Add fragment handles to children of FragmentInstances (facebook#34935)

This PR adds a `unstable_reactFragments?: Set<FragmentInstance>`
property to DOM nodes that belong to a Fragment with a ref (top level
host components). This allows you to access a FragmentInstance from a
DOM node.

This is flagged behind `enableFragmentRefsInstanceHandles`.

The primary use case to unblock is reusing IntersectionObserver
instances. A fairly common practice is to cache and reuse
IntersectionObservers that share the same config, with a map of
node->callbacks to run for each entry in the IO callback. Currently this
is not possible with Fragment Ref `observeUsing` because the key in the
cache would have to be the `FragmentInstance` and you can't find it
without a handle from the node. This works now by accessing
`entry.target.fragments`.

This also opens up possibilities to use `FragmentInstance` operations in
other places, such as events. We can do
`event.target.unstable_reactFragments`, then access
`fragmentInstance.getClientRects` for example. In a future PR, we can
assign an event's `currentTarget` as the Fragment Ref for a more direct
handle when the event has been dispatched by the Fragment itself.

The first commit here implemented a handle only on observed elements.
This is awkward because there isn't a good way to document or expose
this temporary property. `element.fragments` is closer to what we would
expect from a DOM API if a standard was implemented here. And by
assigning it to all top-level nodes of a Fragment, it can be used beyond
the cached IntersectionObserver callback.

One tradeoff here is adding extra work during the creation of
FragmentInstances as well as keeping track of adding/removing nodes.
Previously we only track the Fiber on creation but here we add a
traversal which could apply to a large set of top-level host children.
The `element.unstable_reactFragments` Set can also be randomly ordered.

DiffTrain build for [edd05f1](facebook@edd05f1)

Author: pull[bot]

compiled/facebook-www/REVISION

@@ -1 +1 @@
-561ee24d4a7c805e9837ee4cfbb5671a35e41b5a
+edd05f181b72272fb87b77bd84d2c048a4613955

compiled/facebook-www/REVISION_TRANSFORMS

@@ -1 +1 @@
-561ee24d4a7c805e9837ee4cfbb5671a35e41b5a
+edd05f181b72272fb87b77bd84d2c048a4613955

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

@@ -1499,7 +1499,7 @@ __DEV__ &&
     exports.useTransition = function () {
       return resolveDispatcher().useTransition();
     };
-    exports.version = "19.3.0-www-classic-561ee24d-20251101";
+    exports.version = "19.3.0-www-classic-edd05f18-20251103";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

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

@@ -1499,7 +1499,7 @@ __DEV__ &&
     exports.useTransition = function () {
       return resolveDispatcher().useTransition();
     };
-    exports.version = "19.3.0-www-modern-561ee24d-20251101";
+    exports.version = "19.3.0-www-modern-edd05f18-20251103";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

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

@@ -606,4 +606,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.3.0-www-classic-561ee24d-20251101";
+exports.version = "19.3.0-www-classic-edd05f18-20251103";

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

@@ -606,4 +606,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.3.0-www-modern-561ee24d-20251101";
+exports.version = "19.3.0-www-modern-edd05f18-20251103";

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

@@ -610,7 +610,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.3.0-www-classic-561ee24d-20251101";
+exports.version = "19.3.0-www-classic-edd05f18-20251103";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

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

@@ -610,7 +610,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.3.0-www-modern-561ee24d-20251101";
+exports.version = "19.3.0-www-modern-edd05f18-20251103";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

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

@@ -20393,10 +20393,10 @@ __DEV__ &&
     (function () {
       var internals = {
         bundleType: 1,
-        version: "19.3.0-www-classic-561ee24d-20251101",
+        version: "19.3.0-www-classic-edd05f18-20251103",
         rendererPackageName: "react-art",
         currentDispatcherRef: ReactSharedInternals,
-        reconcilerVersion: "19.3.0-www-classic-561ee24d-20251101"
+        reconcilerVersion: "19.3.0-www-classic-edd05f18-20251103"
       };
       internals.overrideHookState = overrideHookState;
       internals.overrideHookStateDeletePath = overrideHookStateDeletePath;
@@ -20431,7 +20431,7 @@ __DEV__ &&
     exports.Shape = Shape;
     exports.Surface = Surface;
     exports.Text = Text;
-    exports.version = "19.3.0-www-classic-561ee24d-20251101";
+    exports.version = "19.3.0-www-classic-edd05f18-20251103";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

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

@@ -20164,10 +20164,10 @@ __DEV__ &&
     (function () {
       var internals = {
         bundleType: 1,
-        version: "19.3.0-www-modern-561ee24d-20251101",
+        version: "19.3.0-www-modern-edd05f18-20251103",
         rendererPackageName: "react-art",
         currentDispatcherRef: ReactSharedInternals,
-        reconcilerVersion: "19.3.0-www-modern-561ee24d-20251101"
+        reconcilerVersion: "19.3.0-www-modern-edd05f18-20251103"
       };
       internals.overrideHookState = overrideHookState;
       internals.overrideHookStateDeletePath = overrideHookStateDeletePath;
@@ -20202,7 +20202,7 @@ __DEV__ &&
     exports.Shape = Shape;
     exports.Surface = Surface;
     exports.Text = Text;
-    exports.version = "19.3.0-www-modern-561ee24d-20251101";
+    exports.version = "19.3.0-www-modern-edd05f18-20251103";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&