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 <[email protected]>

CONTRIBUTING.md

@@ -199,6 +199,31 @@ the current results, and CI runs `cargo xtask oracle` on every pull request. A c
 because of a known bug names its issue in `expected.json` (`"known_failure": "#123"`); never
 change an expected value to match SolidCraft's output.
 
+## Changing the design file format
+
+A design file says which format it is in (`"format": "solidcraft/N"`). If your change saves
+something new in a design (a field, a feature type, a new kind of name), or makes older designs
+mean something else, it needs a format change. **Add one file, and edit nothing shared:**
+
+1. Create `crates/doc/src/format/pending/<what-it-is>.rs` (lowercase, dashes). Start it with a
+   `//!` note: what is saved, what older designs get without it, and that the last release asks
+   for an update instead of meeting it.
+2. If older designs must be rewritten as they open, put `pub fn step(v: &mut Value) ->
+   crate::Result<()>` in the same file. Pending steps run in file-name order, after the older
+   formats' steps.
+3. Don't touch `FORMAT`, `JSON_STEPS`, the history in `crates/doc/src/format.rs`, another
+   change's file, or any `v<N>/` folder: those are fixed once released.
+
+That's all. The build counts the files: while any are pending it writes the next format number,
+which no release reads, so a release refuses a design that has your change instead of losing it.
+A file also lists the pending changes it has (`format_pending`), so a later development build
+runs only the steps that file is missing. Two pull requests that each add a change merge without
+a conflict. `crates/doc/src/format/pending/README.md` has an example.
+
+**At release time** the maintainer runs `scripts/fold-format-pending.sh`, which moves the pending
+files into `crates/doc/src/format/v<N>/` and fixes that number for good (see
+[docs/releasing.md](docs/releasing.md)). Never add a file to a `v<N>/` folder afterwards.
+
 ## Sending a change
 
 1. Work is tracked in [GitHub issues](https://github.com/bherbruck/solidcraft/issues), labelled