Update changelog for v0.8.21 release

Added release section for October 1, 2025 v0.8.21, including new features and improvements such as strokeAttenuation properties and enhanced type signatures for fromObject methods.

Author: jonobr1

wiki/changelog/README.md

@@ -11,7 +11,9 @@ All notable changes to this project will be documented in this file. The format
 
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-## Nightly
+## October 1, 2025 v0.8.21
+
+<h3 class="visible">Oct 1, 2025</h3><version-link v="v0.8.21" />
 
 - Added `Two.Path.strokeAttenuation`, `Two.Group.strokeAttenuation`, `Two.Points.strokeAttenuation`, `Two.Text.strokeAttenuation`
 - Improved type signatures on all `fromObject` methods for better hinting