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/v24/edge-flange-corner-relief.rs

@@ -0,0 +1,5 @@
+//! An Edge Flange feature can choose its own corner relief (`corner_relief_shape`: notch, round,
+//! rectangular or tear, and `corner_relief`, the size across), for the corners its flanges make.
+//! Version 23 designs need no transformation: a feature without them follows the sheet metal
+//! rule, as it always did, and saves as before; 0.13.0 (which reads up to 22) asks for an update
+//! instead of silently building the rule's relief where a design asks for another.