results: results.xy, a quantity against the load factor, time or mode (#94)

A nodal field at a node per frame (nonlinear static displacements from
load factor 0, explicit frames, heat temperatures) or per mode, the
explicit energies and histories, history nodes, and the frequencies or
buckling load factors; capped at 100000 points with `every`; stale
results refused; `path` writes a two-column CSV. XY Plot is ready.

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

crates/engine/src/cmd/mod.rs

@@ -35,6 +35,7 @@ mod symmetry;
 mod tags;
 pub mod update;
 pub(crate) mod view;
+pub mod xy;
 
 use crate::schema::ProblemKind;
 use fieldcraft_model::MAX_LABEL;
@@ -186,6 +187,7 @@ pub static ALL: &[CommandSpec] = &[
     results::EXPORT_CSV,
     results::EXPORT_VTU,
     results::CLEAR,
+    xy::XY,
     diagnostics::CRASH_REPORTS,
     diagnostics::CRASH_REPORT,
     diagnostics::CRASH_REPORTS_DELETE,