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

crates/mcp/Cargo.toml

@@ -10,6 +10,7 @@ workspace = true
 
 [dependencies]
 fieldcraft-engine = { workspace = true }
+fieldcraft-gpu = { workspace = true }
 fieldcraft-io = { workspace = true }
 fieldcraft-render = { workspace = true }
 serde_json = { workspace = true }