Design format: a change is one new file, never an edit to FORMAT (#1563)

A PR that saves something new adds crates/doc/src/format/pending/<name>.rs (a note, and an upgrade step if older designs must be rewritten). The build writes the next format number while changes are pending and lists them in format_pending; scripts/fold-format-pending.sh fixes them as v<N>/ at release time. Formats 23 and 24 move into v23/ and v24/.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/fold-format-pending.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+# Release step: give the pending design-format changes their number for good.
+#
+# Moves every change in crates/doc/src/format/pending/ into crates/doc/src/format/v<N>/, where N
+# is the number development builds have been writing for them (the last folded format + 1).
+# Commit the result with the release's version bump. Nothing to do when nothing is pending.
+# See crates/doc/src/format/pending/README.md.
+set -euo pipefail
+
+root=$(git rev-parse --show-toplevel)
+dir="$root/crates/doc/src/format"
+last=$(sed -n 's/^const HAND_WRITTEN: u32 = \([0-9]*\);/\1/p' "$root/crates/doc/build.rs")
+[ -n "$last" ] || { echo "HAND_WRITTEN not found in crates/doc/build.rs" >&2; exit 1; }
+for d in "$dir"/v*/; do
+  [ -d "$d" ] || continue
+  n=${d%/}
+  n=${n##*/v}
+  if [ "$n" -gt "$last" ]; then last=$n; fi
+done
+
+shopt -s nullglob
+pending=("$dir"/pending/*.rs)
+if [ ${#pending[@]} -eq 0 ]; then
+  echo "No pending format changes: this release writes format $last."
+  exit 0
+fi
+
+next=$((last + 1))
+mkdir -p "$dir/v$next"
+git -C "$root" mv "${pending[@]}" "$dir/v$next/"
+echo "Format $next is now fixed, with ${#pending[@]} change(s):"
+for f in "${pending[@]}"; do echo "  ${f##*/}"; done
+echo "Commit crates/doc/src/format/ with the version bump."