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/geometry.rs

@@ -1,3 +1,4 @@
+use crate::schema::ProblemKind;
 use std::collections::BTreeSet;
 use std::sync::Arc;
 
@@ -156,14 +157,14 @@ fn bodies(s: &mut Session, _: &Value) -> Result<Value> {
 }
 
 fn body<'a>(s: &'a Session, p: &Value) -> Result<&'a Shape> {
-    let name = p.get("body").and_then(Value::as_str).ok_or_else(|| EngineError::param("body", "must be a body name"))?;
+    let name = p.get("body").and_then(Value::as_str).ok_or_else(|| EngineError::param("body", ProblemKind::NotAName))?;
     s.design.as_deref().and_then(|d| d.body(name)).ok_or_else(|| EngineError::Params(format!("no body `{name}` (open a design first)")))
 }
 
 /// The body named by `body`, tessellated within `tolerance`.
 pub(crate) fn body_mesh<'a>(s: &'a Session, p: &Value) -> Result<(&'a Shape, SurfaceMesh)> {
     let shape = body(s, p)?;
-    let tolerance = p.get("tolerance").and_then(Value::as_f64).ok_or_else(|| EngineError::param("tolerance", "must be a number (mm)"))?;
+    let tolerance = p.get("tolerance").and_then(Value::as_f64).ok_or_else(|| EngineError::param("tolerance", ProblemKind::NotANumber))?;
     Ok((shape, shape.tessellate(tolerance)?))
 }