Engine: model.boundary_condition.symmetry on a datum plane

A symmetry condition (zero normal displacement, zero rotation about the
in-plane axes) on a datum plane normal to X, Y or Z, with its set given or made
of the open design's faces on the plane (a partition's cut faces), so half a
symmetric part is solved as a half. Half a 40 mm bar cut at its mid-width, with
the condition and half the load, bends as the whole bar to 2e-12 with half the
reaction. Inclined planes are refused as not supported yet.

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

crates/engine/src/cmd/mod.rs

@@ -19,6 +19,7 @@ mod model;
 pub(crate) mod octree;
 mod results;
 pub(crate) mod solve;
+mod symmetry;
 mod tags;
 
 use fieldcraft_model::MAX_LABEL;
@@ -96,6 +97,7 @@ pub static ALL: &[CommandSpec] = &[
     define::BOUNDARY_CONDITION_CREATE,
     define::BOUNDARY_CONDITION_EDIT,
     define::BOUNDARY_CONDITION_DELETE,
+    symmetry::SYMMETRY,
     define::LOAD_CREATE,
     define::LOAD_EDIT,
     define::LOAD_DELETE,