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/lib.rs

@@ -41,6 +41,11 @@ pub enum EngineError {
     UnknownCommand(String),
     #[error("{0}")]
     Params(String),
+    /// One parameter is missing or wrong: its key (a JSON path, `elastic/young`) and what is
+    /// wrong with it. The message names the key (agents need it); a form shows the problem
+    /// beside the field, by its label.
+    #[error("`{key}` {problem}")]
+    Param { key: String, problem: String },
     #[error(transparent)]
     Model(#[from] fieldcraft_model::Error),
     #[error(transparent)]
@@ -65,6 +70,13 @@ pub enum EngineError {
     Internal { command: String, message: String },
 }
 
+impl EngineError {
+    /// A problem with parameter `key`.
+    pub fn param(key: impl Into<String>, problem: impl Into<String>) -> Self {
+        Self::Param { key: key.into(), problem: problem.into() }
+    }
+}
+
 pub type Result<T> = std::result::Result<T, EngineError>;
 
 /// The result of a solved step, from the implicit, the explicit or the lattice Boltzmann solver.