fix: re-add LightNode before starting exiting animation (#9646)

## Summary

Fixes #9615 — the `[LA] Reduced Motion` "always reduce" example crashes
on iOS when the box is hidden.

In `startAnimationsRecursively`, the exiting node was re-added to
`lightNodes_` *after* `startExitingAnimation`. On iOS,
`IOSUIScheduler::scheduleOnUI` runs the job synchronously when already
on the main thread, and Fabric mounting runs on the main thread — so
`startExitingAnimation` can re-enter `endLayoutAnimation` before control
returns. With `ReduceMotion.Always` the exiting animation finishes
within the same frame, so the re-entrant `endLayoutAnimation(tag,
/*shouldRemove=*/true)` runs while the node is still missing from
`lightNodes_` (it was erased by the `Delete` mutation and not yet
re-added). `lightNodes_[tag]` then inserts and dereferences a null
`shared_ptr`, crashing.

This is iOS-only because Android queues the job instead of running it
inline, so the re-add happens first.

The fix re-adds the node to `lightNodes_` *before* calling
`startExitingAnimation`, keeping the light tree consistent for the
(possibly re-entrant) `endLayoutAnimation` call.

## Test plan

1. Build the `fabric-example` app on iOS.
2. Open the `[LA] Reduced motion` example.
3. In the `always reduce` section, toggle the box (`show` / `hide`)
several times.
4. The box animates instantly and the app no longer crashes.

Verified on the iOS simulator: before the fix, the first `hide` reliably
crashes; after the fix, repeated show/hide cycles run without crashing
and the box hides correctly.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tomekzaw

packages/react-native-reanimated/Common/cpp/reanimated/LayoutAnimations/LayoutAnimationsProxy_Experimental.cpp

@@ -516,8 +516,8 @@ bool LayoutAnimationsProxy_Experimental::startAnimationsRecursively(
 
   if (hasExitAnimation) {
     node->state = ANIMATING;
-    startExitingAnimation(node);
     lightNodes_[node->current.tag] = node;
+    startExitingAnimation(node);
   } else {
     layoutAnimationsManager_->clearLayoutAnimationConfig(node->current.tag);
   }