engine: friction output of implicit contact

solve.nonlinear counts the slipping nodes, solve.probe gives a secondary node's shear traction
and slip, and the results display draws contact_shear and contact_slip, contact_status coding
stick and slip.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/forms.rs

@@ -883,6 +883,8 @@ pub const CONTOUR: &[Field] = &[
             ("contact_pressure", "Contact pressure"),
             ("contact_gap", "Contact gap"),
             ("contact_status", "Contact status"),
+            ("contact_shear", "Contact shear"),
+            ("contact_slip", "Contact slip"),
             ("none", "None"),
         ],
     )