fix: refuse target-head stranded fragments

actions/changelog-assembled/changelog-assembled.sh

@@ -33,6 +33,12 @@ set -euo pipefail
 # no '## ' heading, and changelog_section extracts the body below HEAD's
 # heading — so the date HEAD stamped into its heading never enters the
 # comparison, and a date difference can never masquerade as a prose one.
+#
+# This guard narrows, but cannot close, the target-movement window: it sees a
+# fragment present when CI reads the target ref, but one can still land after
+# the final run and before merge. Requiring release PRs to be up to date with
+# their target branch before merge is the repository setting that closes that
+# residual window (#253).
 
 base_ref="${1:-${CHANGELOG_ASSEMBLED_BASE:-origin/main}}"
 changelog="${2:-${CHANGELOG:-CHANGELOG.md}}"
@@ -169,6 +175,37 @@ frag_count="$(printf '%s' "$base_frags" | grep -c . || true)"
 
 failures=0
 
+# Refusal: the target branch gained a fragment after this release PR's merge
+# base, so the ceremony could not have consumed it. Merging this tree would
+# strand that fragment for the next release and misattribute when it shipped.
+stranded=""
+while IFS= read -r -d '' entry; do
+  meta="${entry%%$'\t'*}"
+  path="${entry#*$'\t'}"
+  otype="$(printf '%s\n' "$meta" | awk '{ print $2 }')"
+  name="${path##*/}"
+  case "$otype:$name" in
+    blob:README.md) ;;
+    blob:*.md)
+      if ! printf '%s' "$base_frags" | grep -Fxq "$path"; then
+        stranded="${stranded}    ${path}"$'\n'
+      fi
+      ;;
+  esac
+done < <(git ls-tree -z "$base_ref" -- "$dir/")
+if [ -n "$stranded" ]; then
+  {
+    echo "changelog-assembled: fragment(s) on target '$base_ref' were not consumed by this release PR:"
+    echo
+    printf '%s' "$stranded"
+    echo
+    echo "  Merging now would strand these entries for the next release and"
+    echo "  misattribute when they shipped."
+    echo "  Fix: rebase onto the target head and re-run bin/changelog-assemble '$ver'."
+  } >&2
+  failures=$((failures + 1))
+fi
+
 # Refusal: a fragment the ceremony consumed is still present on HEAD. The
 # ceremony deletes exactly what it assembles (#112) — a fragment that
 # survives its own release sits in the directory and is assembled AGAIN