docs(changelog): fold the doc-gate entry into one caller-facing bullet

The separate XmlDocumentationTests bullet named a test class rather than
an observable change, against the "short and user-facing, one tight entry
per change" rule in CONTRIBUTING.md. The prevention outcome now rides on
the CS1570 entry as a clause; the test detail lives in the PR body.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: marius-bughiu

CHANGELOG.md

@@ -6,8 +6,7 @@ All notable changes to Celerity are documented here. This project follows [Keep
 
 ### Added
 
-- **CS1570 (badly formed XML in a doc comment) is now a build error** in all seven shipping packages, alongside the existing CS1591 gate. A malformed comment is not truncated by the doc writer — the whole member is dropped from the shipped `.xml` — so a warning was too weak a signal. Closes [#356](https://github.com/marius-bughiu/Celerity/issues/356).
-- `XmlDocumentationTests` — asserts each shipped `.xml` parses and carries an entry for every public type of its assembly, so a type cannot silently lose its documentation again. Closes [#356](https://github.com/marius-bughiu/Celerity/issues/356).
+- **CS1570 (badly formed XML in a doc comment) is now a build error** in all seven shipping packages, alongside the existing CS1591 gate, and each package's shipped `.xml` is asserted to carry an entry for every public type. A malformed comment is not truncated by the doc writer — the whole member is dropped — so a warning was too weak a signal for something that silently empties a type's documentation. Closes [#356](https://github.com/marius-bughiu/Celerity/issues/356).
 
 ### Fixed