docs: CHANGELOG — attribute arm64 work to @wetneb, fold CI under Changes

Two cosmetic touch-ups to the 0.3.11 release notes:

* Add `(thanks @wetneb)` attribution to the arm64 portability fix
  and the arm64 CI job, matching the contributor-credit convention
  used in 0.3.5 and 0.3.6 entries.
* Fold the `### CI` subsection under `### Changes` so the prose
  shape matches earlier 0.3.x entries (the CI-only subsection was
  a new category with no prior precedent in this CHANGELOG).

Author: dginev

CHANGELOG.md

@@ -18,16 +18,14 @@
 * Whole-crate clippy pass: `cargo clippy --fix --all-targets` plus
   manual touch-ups so the workspace builds warning-free on the
   current stable.
+* Added an arm64 build/test job to `.github/workflows/CI.yml`
+  (thanks @wetneb).
 
 ### Fixed
 
 * Test suite portability on ARM64 — switched to portable types in
   `tests/xml_copy_invariant_tests.rs` so the cross-platform CI run
-  (new arm64 GitHub Actions job) passes alongside x86_64.
-
-### CI
-
-* Added an arm64 build/test job to `.github/workflows/CI.yml`.
+  passes alongside x86_64 (thanks @wetneb).
 
 ## [0.3.10] (2026-05-09)