ci: anchor changelog insert above latest release

Author: zfarrell

.github/workflows/regenerate.yml

@@ -78,8 +78,9 @@ jobs:
       # check-release.yml gates merges on a `## [x.y.z]` CHANGELOG section
       # matching the bumped version, so without a seeded entry every regen PR
       # fails that check. Insert a stub (the spec-change title under ### Changed)
-      # right after `## [Unreleased]`; the PR author refines the wording before
-      # merge. Idempotent: skips if a section for this version already exists.
+      # just above the most recent released section; the PR author refines the
+      # wording before merge. Idempotent: skips if a section for this version
+      # already exists.
       - name: Seed changelog entry
         env:
           VERSION: ${{ steps.pkg.outputs.version }}
@@ -97,13 +98,16 @@ jobs:
           if re.search(rf"^## \[{re.escape(version)}\]", text, re.M):
               print(f"CHANGELOG already has a [{version}] section; leaving it untouched.")
               raise SystemExit(0)
-          marker = "## [Unreleased]\n"
-          idx = text.find(marker)
-          if idx == -1:
+          unreleased = re.search(r"^## \[Unreleased\]", text, re.M)
+          if not unreleased:
               raise SystemExit("CHANGELOG.md has no '## [Unreleased]' section to anchor the new entry")
-          insert_at = idx + len(marker)
-          entry = f"## [{version}] - {date}\n\n### Changed\n\n- {title}\n"
-          text = text[:insert_at] + "\n" + entry + "\n" + text[insert_at:].lstrip("\n")
+          # Insert before the first released section after [Unreleased] (falling
+          # back to end of file) so any pending entries under [Unreleased] stay
+          # attributed to it rather than being absorbed by the new version.
+          nxt = re.search(r"^## \[", text[unreleased.end():], re.M)
+          insert_at = unreleased.end() + nxt.start() if nxt else len(text)
+          entry = f"## [{version}] - {date}\n\n### Changed\n\n- {title}\n\n"
+          text = text[:insert_at] + entry + text[insert_at:]
           path.write_text(text)
           print(f"Inserted CHANGELOG [{version}] section.")
           PY