chore: reword changeset to reflect bug-fix framing

The refactor closes a 5-year-old correctness bug (#6733), not just a
perf optimization. Update the changeset entry to match.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.changeset/flip-batched.md

@@ -2,4 +2,4 @@
 'svelte': patch
 ---
 
-perf: batch FLIP animation reads and writes to eliminate per-item layout thrash
+fix: capture FLIP sizes before any sibling leaves flex flow