cli, mcp: snapshot --section and --field; headless MCP screenshots on the GPU

--section x|y|z[=VALUE] hides the elements on the camera's side of a plane
(at the mesh's middle by default) so the element faces inside show;
--field contours a nodal field of the solved step (results.contour). The
MCP server's headless screenshot draws with the viewport's pipeline on the
shared device and its reply, like snapshot's, names the renderer.

Refs #422

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

xtask/src/layers.rs

@@ -71,6 +71,8 @@ pub const TABLE: &[(&str, Class)] = &[
 pub const INTRA_LAYER_ORDER: &[&[&str]] = &[
     // The window draws through the GPU crate's pipelines.
     &["gpu", "ui-egui"],
+    // The MCP server's headless screenshots draw with them too.
+    &["gpu", "mcp"],
 ];
 
 fn intra_layer_allowed(from: &str, to: &str) -> bool {