docs: add full Unicode support to CHANGELOG

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jwaldrip

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Changelog
 
+## Unreleased
+
+### Features
+
+* **spec:** Add full Unicode support per GraphQL September 2025 specification ([#1405](https://github.com/absinthe-graphql/absinthe/pull/1405))
+  - Variable-width Unicode escape sequences: `\u{XXXXXX}`
+  - Surrogate pair handling for legacy `\uXXXX` escapes
+  - Validation for Unicode scalar values
+  - Reject lone surrogates
+
 ## [1.9.0](https://github.com/absinthe-graphql/absinthe/compare/v1.8.0...v1.9.0) (2025-11-21)