engine: contact output of nonlinear static steps

solve.nonlinear summarises each contact (normal force, largest pressure, closed nodes, deepest
penetration, augmentations), solve.probe gives a secondary node's pressure, gap and status per
contact, and the results display draws contact_pressure, contact_gap and contact_status. The
contact commands, the catalogue and the model docs say what nonlinear static steps do with a
pair.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/forms.rs

@@ -880,6 +880,9 @@ pub const CONTOUR: &[Field] = &[
             ("stress_xy", "Stress XY"),
             ("stress_xz", "Stress XZ"),
             ("stress_yz", "Stress YZ"),
+            ("contact_pressure", "Contact pressure"),
+            ("contact_gap", "Contact gap"),
+            ("contact_status", "Contact status"),
             ("none", "None"),
         ],
     )