Rust: Document trait item canonical path change

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 5a15104d-a9a3-4506-8325-203af6db47f8

Author: hvitved

rust/ql/lib/change-notes/2026-08-19-trait-canonical-paths.md

@@ -0,0 +1,6 @@
+---
+category: minorAnalysis
+---
+* Canonical paths for Rust trait items now use the format `crate::Trait::item` instead of
+  `<_ as crate::Trait>::item`. Custom data extension models that reference trait items
+  must be updated to use the new format.