add changelong entry

Author: KRRT7

CHANGELOG.md

@@ -4,6 +4,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Updated
+- Speed up `validate_gantt` function by 58x via optimized DataFrame iteration by codeflash [[#5386](https://github.com/plotly/plotly.py/pull/5386)]
+
 ## [6.5.0] - 2025-11-17
 
 ### Updated