Engine: a parameter error carries its key and problem (EngineError::Param)

A form can show the problem beside the field it is about. The message
still names the key, as before, for agents.

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

crates/engine/src/cmd/solve.rs

@@ -134,7 +134,7 @@ fn cap<T: Copy + TryInto<u64> + TryFrom<u64> + std::fmt::Display>(p: &Value, key
             .as_u64()
             .filter(|n| (1..=most).contains(n))
             .and_then(|n| T::try_from(n).ok())
-            .ok_or_else(|| EngineError::Params(format!("`{key}` must be a whole number from 1 to {default}"))),
+            .ok_or_else(|| EngineError::param(key, format!("must be a whole number from 1 to {default}"))),
     }
 }
 
@@ -390,7 +390,7 @@ fn history(s: &mut Session, p: &Value) -> Result<Value> {
     let h = &r.history;
     let samples = h.time.len();
     let every = match opt::<u64>(p, "every")? {
-        Some(0) => return Err(EngineError::Params("`every` must be at least 1".into())),
+        Some(0) => return Err(EngineError::param("every", "must be at least 1")),
         Some(k) => usize::try_from(k).unwrap_or(usize::MAX),
         None => samples.div_ceil(MAX_HISTORY_SAMPLES).max(1),
     };
@@ -541,7 +541,7 @@ fn probe_fluid(r: &FluidResult, p: &Value) -> Result<Value> {
 }
 
 fn probe(s: &mut Session, p: &Value) -> Result<Value> {
-    let step = opt::<String>(p, "step")?.ok_or_else(|| EngineError::Params("`step` is required".into()))?;
+    let step = opt::<String>(p, "step")?.ok_or_else(|| EngineError::param("step", "is required"))?;
     let result = s.result(&step).ok_or_else(|| {
         EngineError::Params(if s.latest_result(&step).is_some() {
             format!("the result of step `{step}` is stale (the model changed since it was solved); solve it again")