update changelog

Author: ytdHuang

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased](https://github.com/qutip/QuantumToolbox.jl/tree/main)
 
+- Align statistical analysis methods for multi-trajectory solutions to `QuTiP` [#471]. Remove field `average_expect`, replace field `runs_expect` as `expect`, and introduce the following functions:
+  - `average_states`
+  - `average_expect`
+  - `std_expect`
+
 ## [v0.31.1]
 Release date: 2025-05-16
 
@@ -228,3 +233,4 @@ Release date: 2024-11-13
 [#455]: https://github.com/qutip/QuantumToolbox.jl/issues/455
 [#456]: https://github.com/qutip/QuantumToolbox.jl/issues/456
 [#460]: https://github.com/qutip/QuantumToolbox.jl/issues/460
+[#471]: https://github.com/qutip/QuantumToolbox.jl/issues/471