chore(release): restore the living [3.8.51] CHANGELOG section

The cycle-open commit's root CHANGELOG hunk was lost in the rebase onto the
branch tip (the 42 i18n mirrors kept theirs, so the root and the mirrors had
drifted apart). Re-inserts the section with the three canonical headings.

CHANGELOG.md

@@ -16,6 +16,18 @@
 
 ---
 
+## [3.8.51] — TBD
+
+_Living section — cycle opened at the v3.8.50 freeze (parallel-cycle model). Bullets are aggregated from `changelog.d/` fragments at each `/generate-release` phase._
+
+### ✨ New Features
+
+### 🐛 Bug Fixes
+
+### 📝 Maintenance
+
+---
+
 ## [3.8.50] — TBD
 
 _Living section — regenerated 2026-08-12 from all cycle commits (cycle open `ed2db6cb19` → tip). Bullets carry the merged PR and its author; direct pushes listed separately._