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>

.github/workflows/lists.yml

@@ -0,0 +1,20 @@
+# Files many pull requests add to are kept so that additions don't conflict: one file per change
+# or one sorted line per item (CONTRIBUTING.md, "Lists many changes add to"). Fast, no build.
+name: Lists
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  format-changes:
+    name: Design format changes merge cleanly
+    runs-on: ubuntu-24.04
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - run: bash scripts/test-format-pending.sh