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/forms.rs

@@ -716,6 +716,13 @@ pub const MIRROR: &[Field] = &[
     json("as", "Images").hint(r#"{"body": "image body"}; default: the design's body at the reflected place"#),
 ];
 
+pub const SYMMETRY: &[Field] = &[
+    NAME,
+    reference("datum", "Datum plane", C::DatumPlane).req(),
+    reference("set", "Set", C::Set).hint("the faces of the design on the plane"),
+    references("bodies", "Bodies", C::Body).hint("every body"),
+];
+
 pub const HEX_OCTREE: &[Field] = &[
     BODY,
     SURFACE,