Move Expr.__add__/__iadd__ note to Changed

Relocate the "Speed up `Expr.__add__` and `Expr.__iadd__` via the C-level API" bullet from the 6.2.1 section into the top-level "Changed" section of the changelog to remove duplication and better reflect its categorization.

Author: Zeroto521

CHANGELOG.md

@@ -4,6 +4,7 @@
 ### Added
 ### Fixed
 ### Changed
+- Speed up `Expr.__add__` and `Expr.__iadd__` via the C-level API
 ### Removed
 
 ## 6.2.1 - 2026.05.16
@@ -31,7 +32,6 @@
 - Return NotImplemented for `Expr` and `GenExpr` operators if they can't handle input types in the calculation
 - Speed up `constant * Expr` via C-level API
 - Speed up `Term.__eq__` via the C-level API
-- Speed up `Expr.__add__` and `Expr.__iadd__` via the C-level API
 ### Removed
 - Removed outdated warning about Make build system incompatibility
 - Removed `Term.ptrtuple` to optimize `Term` memory usage