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>

crates/doc/src/format/v23/import-fcstd-3dm.rs

@@ -0,0 +1,4 @@
+//! An Import's `file` can name a FreeCAD `.FCStd` or Rhino `.3dm` file (#1435, read through
+//! cadmpeg); its `step` is still the STEP text the file was restated as, so the feature's fields
+//! are unchanged and version 22 designs need no transformation; 0.13.x asks for an update before
+//! it meets these imports.

crates/doc/src/format/v23/motion-link-coupling.rs

@@ -0,0 +1,7 @@
+//! A motion link can carry a `coupling`: a gear pair (`ra`, `rb`, `internal`), a rack and pinion
+//! (`radius`) or a screw (`pitch`), each size a number in mm, an expression or a picked round
+//! face, and `reverse`. The link's `ratio` is worked out from it whenever the design changes and
+//! saved as well, so a link reads the same without it. Version 22 designs need no
+//! transformation: a link without `coupling` is a plain motion link, as it always was; 0.13.0
+//! (which reads up to 22) asks for an update instead of keeping a gear's ratio when its radius
+//! changes.