docs: node relaxation in features, MCP guide, control protocol, literature and the meshing skill (#398)

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

crates/engine/src/cmd/forms.rs

@@ -1891,6 +1891,7 @@ mod tests {
         "has a result",
         "no crash report",
         "FIELDCRAFT_CRASH_TEST",
+        "no relaxation field",
     ];
 
     /// Fill top-level required names (a fresh one) and references (the first existing item).

crates/engine/src/cmd/relax.rs

@@ -374,7 +374,7 @@ pub(crate) fn weights(model: &Model, p: &Value) -> Result<(BTreeMap<u32, f64>, O
         nodes.extend(el.nodes.iter().copied());
     }
     if listed == 0 {
-        return Err(EngineError::Params("say which nodes to relax: `center` and `radius`, `around_element`, `nodes`, `elements` or `set`".into()));
+        return Err(EngineError::Params("say which nodes to relax: give one of `center` and `radius`, `around_element`, `nodes`, `elements` or `set`".into()));
     }
     if let Some(only) = &only {
         nodes.retain(|n| only.contains(n));