CHANGELOG: seal the 2026.9.18.1 section

release.yml extracts a release's notes from CHANGELOG.md by scanning for a
"## [<version>]" heading; the merged PR left this version's entries under
"## [Unreleased]" only, so the just-published v2026.9.18.1 GitHub Release
went out with empty notes ("(no CHANGELOG entry found for 2026.9.18.1)").
Adds the version heading above the existing content (nothing in the entries
themselves changes) and reopens an empty [Unreleased] section above it, per
this file's own Keep a Changelog convention. The release's notes are
corrected separately, after this lands, by re-running the same extraction
by hand.

Author: speak-agent

CHANGELOG.md

@@ -5,6 +5,8 @@
 
 ## [Unreleased]
 
+## [2026.9.18.1] - 2026-09-18
+
 ### C 库层可以声明它呈现的 C 环境,引擎实现并校验:设计 2026-09-18(2026.9.18.1)
 
 三元组的 OS 段过去总是同时回答两件事:目标机器长什么样,以及源码面对的 C 环境是什么。