feat: enforce changelog shape in guard and assembler

actions/changelog-armed/changelog-armed.sh

@@ -96,6 +96,11 @@ if [ -d "$fragments_dir" ]; then
     fi
   done <<<"$fragments"
 
+  if ! diagnosis="$(changelog_shape_problem "$changelog" "$fragments_dir")"; then
+    printf 'changelog-armed: %s\n' "$diagnosis" >&2
+    exit 1
+  fi
+
   if version_is_dev "$ver"; then
     echo "changelog-armed: version '$ver' agrees with fragment mode ($fragments_dir)"
     exit 0

bin/changelog-assemble

@@ -76,6 +76,10 @@ done
 fragments="$(changelog_fragments "$dir")"
 [ -n "$fragments" ] || refuse "zero fragments in '$dir' — a release publishes prose; refusing to publish an empty release"
 
+if ! diagnosis="$(changelog_shape_problem "$changelog" "$dir")"; then
+  refuse "$diagnosis"
+fi
+
 if ! body="$(changelog_assemble "$dir")"; then
   refuse "$body"
 fi

test/changelog-armed.test.sh

@@ -296,6 +296,66 @@ EOF
 check "fragment -dev + well-formed grouped fragment passes" 0 "fragment mode" \
   in_tree fragments-dev-grouped
 
+fragment_tree fragments-dev-mixed 1.2.4-dev <<'EOF'
+# Changelog
+
+## 1.2.3 — 2026-07-20
+
+- The shipped entry.
+EOF
+printf '%s\n' "- Flat fragment." >"$TMP/fragments-dev-mixed/changelog.d/114.md"
+cat >"$TMP/fragments-dev-mixed/changelog.d/115.md" <<'EOF'
+### Fixed
+
+- Grouped fragment.
+EOF
+check "fragment mode refuses mixed shapes with the shared assembler diagnosis" 1 \
+  "fragment 'changelog.d/115.md' is grouped but fragment 'changelog.d/114.md' is not" \
+  in_tree fragments-dev-mixed
+
+fragment_tree fragments-dev-all-grouped-over-flat 1.2.4-dev <<'EOF'
+# Changelog
+
+## 1.2.3 — 2026-07-20
+
+- The shipped entry.
+EOF
+cat >"$TMP/fragments-dev-all-grouped-over-flat/changelog.d/115.md" <<'EOF'
+### Fixed
+
+- Grouped fragment.
+EOF
+check "fragment mode refuses an all-grouped set over a flat published section" 1 \
+  "changelog.d/115.md' is grouped but newest published section '1.2.3'" \
+  in_tree fragments-dev-all-grouped-over-flat
+
+fragment_tree fragments-dev-flat-over-grouped 1.2.4-dev <<'EOF'
+# Changelog
+
+## 1.2.3 — 2026-07-20
+
+### Fixed
+
+- The shipped entry.
+EOF
+printf '%s\n' "- Flat fragment." >"$TMP/fragments-dev-flat-over-grouped/changelog.d/115.md"
+check "fragment mode refuses a flat set over a grouped published section" 1 \
+  "changelog.d/115.md' is flat but newest published section '1.2.3'" \
+  in_tree fragments-dev-flat-over-grouped
+
+fragment_tree fragments-dev-no-published 1.2.4-dev <<'EOF'
+# Changelog
+
+Preamble only.
+EOF
+cat >"$TMP/fragments-dev-no-published/changelog.d/115.md" <<'EOF'
+### Fixed
+
+- Grouped fragment.
+EOF
+check "fragment mode accepts a consistent set with no published section" 0 \
+  "fragment mode" in_tree fragments-dev-no-published
+
 fragment_tree fragments-unreleased 1.2.4-dev <<'EOF'
 # Changelog
 

test/changelog-assemble.test.sh

@@ -99,8 +99,16 @@ check "flat: numeric-descending order (10.md before 9.md), cross-repo name besid
 
 # --- grouped write: canonical order, unnamed group appended ------------------
 
-tree grouped <<EOF
-$BASE_CHANGELOG
+tree grouped <<'EOF'
+# Changelog
+
+Preamble prose belongs to no section.
+
+## 0.1.0 — 2026-07-01
+
+### Fixed
+
+- The shipped entry.
 EOF
 frag grouped 21.md <<'EOF'
 ### Fixed
@@ -133,7 +141,7 @@ check "grouped: write mode assembles the same section" 0 "consumed 3 fragment" \
   in_tree grouped 0.2.0 2026-07-24
 check "grouped: the written file is exact" 0 "" \
   assert_file "$TMP/grouped/CHANGELOG.md" \
-  $'# Changelog\n\nPreamble prose belongs to no section.\n\n## 0.2.0 — 2026-07-24\n\n'"$GROUPED_BODY"$'\n\n## 0.1.0 — 2026-07-01\n\n- The shipped entry.'
+  $'# Changelog\n\nPreamble prose belongs to no section.\n\n## 0.2.0 — 2026-07-24\n\n'"$GROUPED_BODY"$'\n\n## 0.1.0 — 2026-07-01\n\n### Fixed\n\n- The shipped entry.'
 
 # --- a changelog holding only its preamble -----------------------------------
 
@@ -281,6 +289,18 @@ check "grouped + flat mixed refuses, both files named" 1 "6.md" \
 check "the mixed refusal names the flat side too" 1 "5.md" \
   in_tree mixed 0.2.0
 
+tree grouped-over-flat <<EOF
+$BASE_CHANGELOG
+EOF
+frag grouped-over-flat 6.md <<'EOF'
+### Added
+
+- Grouped six.
+EOF
+check "an all-grouped set over a flat published section refuses before assembly" 1 \
+  "fragment 'changelog.d/6.md' is grouped but newest published section '0.1.0'" \
+  in_tree grouped-over-flat 0.2.0
+
 tree already <<'EOF'
 # Changelog
 
@@ -324,8 +344,16 @@ check "--dir without a value is a usage error" 2 "usage:" in_tree flat-one 0.2.0
 
 # --- round trip: the publisher and the assembler agree by test ---------------
 
-tree round-trip <<EOF
-$BASE_CHANGELOG
+tree round-trip <<'EOF'
+# Changelog
+
+Preamble prose belongs to no section.
+
+## 0.1.0 — 2026-07-01
+
+### Fixed
+
+- The shipped entry.
 EOF
 frag round-trip 30.md <<'EOF'
 ### Added