Engine: EngineError::Param carries the schema Problem at its field's path

The ~40 parameter checks give a ProblemKind instead of text; Session::execute looks the problem
up in the command's form (a group's missing field becomes that field's own path). Replies still
name keys; forms phrase the same problem by label. A test refuses every field of every form and
checks each refusal names a field of that form.

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

crates/engine/src/cmd/contact.rs

@@ -12,6 +12,7 @@
 //! touch there. Each touching pair is reported with the faces of each side that are close to the
 //! other; with `create`, each becomes two face sets and a contact pair.
 
+use crate::schema::ProblemKind;
 use std::collections::BTreeMap;
 
 use fieldcraft_fem::contact::{Grid, bounds, closest_point};
@@ -34,7 +35,7 @@ fn contact(p: &Value, surfaces: ContactSurfaces) -> Result<Contact> {
 
 /// A required name-valued parameter.
 fn text<'a>(p: &'a Value, key: &str) -> Result<&'a str> {
-    p.get(key).and_then(Value::as_str).ok_or_else(|| EngineError::param(key, "must be a set name"))
+    p.get(key).and_then(Value::as_str).ok_or_else(|| EngineError::param(key, ProblemKind::NotAName))
 }
 
 pub const PAIR: CommandSpec = CommandSpec {