results: clamp the contour legend's range; values above it in an over colour (#191)

results.contour takes range {min?, max?} (or "auto"), kept until the field changes: a
singular peak no longer squeezes the field into the bottom bands. Values above a clamped
maximum go to an extra band drawn in the over colour, split exactly at the clamp; the legend
shows that band up to the true maximum and says it is clamped. The results panel has Legend
max/min (Auto or a value), the MCP show_results tool a range argument, and headless pictures
follow the clamp too.

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

crates/engine/src/cmd/forms.rs

@@ -1670,6 +1670,15 @@ pub const CONTOUR: &[Field] = &[
     .req()
     .def("\"von_mises\""),
     int("bands", "Bands", 2, 24).help("Number of colour bands.").def("12"),
+    group(
+        "range",
+        "Legend range",
+        &[
+            num("min", "Min", D::None).help("Values below it take the lowest colour.").hint("the field's minimum"),
+            num("max", "Max", D::None).help("Values above it are drawn in the over colour.").hint("the field's maximum"),
+        ],
+    )
+    .help("Clamp the legend, e.g. below a singular peak; empty follows the field."),
 ];
 pub const MODES: &[Field] = &[SOLVED, int("mode", "Mode", 1, 1_000_000).help("The mode shape shown.").def("1")];
 pub const ANIMATE: &[Field] = &[flag("on", "Animate").help("Play the deformation or the frames.").def("true")];