Sheet metal: tear as a fourth corner relief shape (#809)

A sheet metal rule's corner relief shape can now be tear; saved under format 17 with a note.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/format.rs

@@ -97,6 +97,9 @@
 //!   A fillet's `type` can be `spline` (the radius along each edge a B-spline: `start`, `knots`,
 //!   and the control radii after `radius` in `radii`). Other fillets save as before; 0.8.0 would
 //!   refuse the unknown type.
+//!   Under the same version, a sheet metal rule's corner relief shape can also be `tear` (no
+//!   relief: each bend is cut straight from the bends' corner to where its flange's end
+//!   starts); a rule that says `notch`, `round` or `rectangular` builds what it did.
 
 use serde_json::Value;