feat: corner reliefs at sheet flange corners, chosen by the rule (#488)

Sheet Metal Rules get a Corner Relief Shape (notch, round or rectangular), sized by the rule's corner relief, cut where two bends meet at a corner of the base. Notch, the default, keeps existing designs as they were. File format 16.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/book/src/guide/files.md

@@ -51,7 +51,7 @@ keeps the current design. Previews cannot be edited or measured. This experiment
 requires `preview: true` in an engine Save/Save As call; ordinary saves omit display caches.
 The repository's `docs/design-container.md` describes limits and encoding details.
 
-Current designs use format 15, including sketch face anchors on cylinder caps, sketch axis
+Current designs use format 16, including sheet metal corner relief shapes, sketch face anchors on cylinder caps, sketch axis
 links, persistent
 names from exact face history, exact planar shell-face anchors, persistent analytic offset boundary roles and
 association profile references, durable sketch face anchors, periodic fit splines and versioned

docs/book/src/guide/sheet-metal.md

@@ -14,6 +14,25 @@ The SHEET METAL tab builds parts from a sheet of constant thickness under a **sh
 Bends follow the rule's K-factor, so flat lengths match what a press brake will make. Tests
 check flat lengths against hand-computed bend allowances.
 
+Where two bends of one edge flange feature meet at a corner of the base, the rule's **corner
+relief shape** (`corner_relief_shape`; Corner Relief Shape in Sheet Metal Rules) says what is
+cut there. `notch`, the default, cuts nothing more: the bends end square at the corner. `round`
+and `rectangular` cut a relief the rule's **corner relief** across, centred on the corner where
+the two bend lines meet: out of the base a round with half that size as its radius, or a square
+with half that size as its side, and both bends stop where it begins, cut straight across. The
+flange panels keep their ends. The relief is in the folded part, the flat pattern, the bend
+table (the bends are shorter) and the DXF (a round relief is an arc). At a corner that isn't
+square the round follows the angle of the base and the square becomes a kite, its two cuts
+square to the edges. Changing the shape in a rule changes the parts made with that rule. A
+relief that doesn't fit (under 0.01 mm across, leaving nothing of a bend, or reaching as far
+from its corner as another edge of the base is) is left out, and the flange feature reports a
+warning saying where.
+
+```json
+{"command":"sheet.manage_rules","params":{"name":"Sheet 2 mm","thickness":"2 mm","k_factor":0.5,
+ "bend_radius":"1 mm","corner_relief_shape":"round","corner_relief":"8 mm","active":true}}
+```
+
 Where the edge flanges of one feature meet at a corner of the base, their panels are closed
 toward each other at any bend angle: each end is cut square to the sheet, on a line in the flat
 chosen so that the edges nearest each other (on the inner faces) run parallel, the rule's

docs/design-container.md

@@ -7,7 +7,7 @@ self-contained input/output format. `io::write_design` keeps its JSON contract;
 `io::write_design_container` is the fallible container writer.
 
 Packages have `container.json` with `{"format":"solidcraft-container/1"}` and
-`document.json` with the versioned parametric document (currently `solidcraft/15`).
+`document.json` with the versioned parametric document (currently `solidcraft/16`).
 The independent versions let document migrations evolve without changing blob encodings.
 Existing serialized document fields are retained automatically, without a second copy
 of the document schema or a deep clone of embedded payloads.