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 <noreply@anthropic.com>

crates/engine/src/cmd/sheet.rs

@@ -36,7 +36,9 @@ pub static COMMANDS: &[CommandSpec] = &[
         .params("body; face: [x,y,z] on its large flat face (a plate of even thickness); rule?"),
     CommandSpec::new("sheet.manage_rules", "Sheet Metal Rules", rules).at("SHEET METAL", "MODIFY").icon("sheet_rules").params(
         "name (new or existing); thickness?, k_factor?, bend_radius?, relief_width?, relief_depth?, corner_relief?, hem_gap?, gap? (expressions; `Thickness` is the rule's; \
-         gap: between flanges that meet at a corner, whose ends are closed toward each other at any angle); active?: bool; delete?: bool",
+         gap: between flanges that meet at a corner, whose ends are closed toward each other at any angle); \
+         corner_relief_shape?: notch|round|rectangular (what is cut where two bends meet at a corner of the base, `corner_relief` across; default notch); \
+         active?: bool; delete?: bool",
     ),
     CommandSpec::new("sheet.rules", "List Sheet Metal Rules", list_rules).noundo().doc_only().params("→ rules with their expressions and values, and the active one"),
     CommandSpec::new("sheet.flat_pattern", "Create Flat Pattern", flat_pattern)
@@ -269,6 +271,10 @@ fn rules(s: &mut Session, p: &Value) -> Result<Value> {
             *slot = e;
         }
     }
+    if let Some(shape) = p.get("corner_relief_shape").filter(|v| !v.is_null()) {
+        let shape = shape.as_str().and_then(solidcraft_doc::sheet::ReliefShape::parse);
+        rule.corner_relief_shape = shape.ok_or_else(|| bad(cmd, "`corner_relief_shape` must be notch, round or rectangular"))?.name().into();
+    }
     let (vals, _) = s.doc.param_values();
     let values = s.doc.rule_values(&vals, &rule).map_err(|e| bad(cmd, e.to_string()))?;
     let doc = s.doc_mut();