fix(changelog): one fragment, one diagnosis, wherever the long entry sits

awk runs END on the way out of an exit from a main rule, so the length
row printed mid-file was followed by the citation row it outranks — the
internal protocol line landing inside the human-facing excerpt. Found by
claude-bot and kimi-bot in #262's first round, independently and with the
same reproduction.

The guard is the reported flag the empty-heading walk in this same
predicate already uses. The fixtures are the axis 57.md could not reach:
its over-bound entry is last, so only END's flush can print. 58.md puts
one before another bullet, 59.md before a heading and after a misplaced
cite. With lib/changelog.sh alone reverted they red, which is what the
green suite was hiding.

Refs #262.

changelog.d/262.md

@@ -7,6 +7,9 @@
 - The refusal distinguishes an entry carrying no reference at all from one
   whose reference is present but not terminal, and names the shape to
   write in both (#262).
+- The 300-character bound still outranks the citation across the whole
+  fragment, and the outranked problem stays out of the message it lost
+  to: one fragment, one diagnosis, wherever in the file it sits (#262).
 
 ### Changed
 

lib/changelog.sh

@@ -208,6 +208,7 @@ changelog_fragment_problem() {
         sub(/ $/, "", e)
         len = length(e)
         if (len > max) {
+          reported = 1
           printf "long\t%d\t%s\n", len, excerpt(e)
           return 1
         }
@@ -227,9 +228,15 @@ changelog_fragment_problem() {
       }
       /^[[:space:]]*$/ { next }
       entry != "" { entry = entry " " $0 }
+      # An exit from a main rule still runs END, so a length row printed
+      # mid-file would be followed by the citation row it outranks — two
+      # lines spliced into one diagnosis, the internal protocol row landing
+      # inside the human-facing excerpt (#262 round 1). The reported flag is
+      # the same guard the empty-heading walk above uses, for the same
+      # reason: one diagnosis per fragment is the contract.
       END {
         if (flush()) exit
-        if (cite_kind != "") printf "%s\t\t%s\n", cite_kind, cite_excerpt
+        if (!reported && cite_kind != "") printf "%s\t\t%s\n", cite_kind, cite_excerpt
       }
     ' "$file"
   )"

test/changelog.test.sh

@@ -420,16 +420,44 @@ check "cite: the reference need not match the filename" 0 "" \
 check "cite: an over-bound entry outranks an earlier uncited one" 1 \
   "57.md' has a 301-character entry" \
   changelog_fragment_problem "$PF/57.md"
-assert_one_diagnosis_57() {
+assert_one_diagnosis() {
   local count
-  count="$(changelog_fragment_problem "$PF/57.md" | wc -l)"
+  count="$(changelog_fragment_problem "$PF/$1.md" | wc -l)"
   [ "$count" = 1 ] || {
     printf 'wanted one diagnosis, got %s\n' "$count"
     return 1
   }
 }
 check "cite: the outranked citation problem is not reported beside it" 0 "" \
-  assert_one_diagnosis_57
+  assert_one_diagnosis 57
+
+# The axis 57.md cannot test: its over-bound entry is LAST, so the only
+# flush that can print is END's, which exits immediately. A flush from a
+# main rule exits too — but awk runs END on the way out, so the citation
+# row a mid-file length row outranks would print after it unless END is
+# guarded. Both ways out of the walk, a bullet and a heading.
+{
+  printf -- '- Uncited, and it comes first.\n'
+  printf -- '- %s\n' "$(mkchars 301)"
+  printf -- '- A later entry the walk never reaches (#57).\n'
+} >"$PF/58.md"
+check "cite: an over-bound entry that is not the last one still reports the bound" 1 \
+  "58.md' has a 301-character entry" \
+  changelog_fragment_problem "$PF/58.md"
+check "cite: and it is still one diagnosis, not the protocol row spliced into it" 0 "" \
+  assert_one_diagnosis 58
+{
+  printf '### Fixed\n'
+  printf -- '- Misplaced, and it comes first. (#59)\n'
+  printf -- '- %s\n' "$(mkchars 301)"
+  printf '### Changed\n'
+  printf -- '- The heading is the other way out of the walk (#59).\n'
+} >"$PF/59.md"
+check "cite: a heading after the over-bound entry is the same one diagnosis" 1 \
+  "59.md' has a 301-character entry" \
+  changelog_fragment_problem "$PF/59.md"
+check "cite: the misplaced row does not ride along with it either" 0 "" \
+  assert_one_diagnosis 59
 
 # Published sections keep their pre-rule prose (D4): reddening history is a
 # wall, not a guard. Every shipped section predates the cite.