solver, decks: symmetry about an inclined plane (#242)

The symmetry condition takes any plane normal ({"normal": [nx, ny, nz]}).
An inclined one holds n·u = 0 and, at shell, beam and reference point
nodes, the rotations about the two in-plane axes: constraint rows the
implicit elimination already handles, its support forces reported as
reactions. Decks: Abaqus *EQUATION, Nastran MPC sets per subcase, LS-DYNA
*BOUNDARY_SPC in a *DEFINE_COORDINATE_SYSTEM along the normal.

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

crates/model/src/check.rs

@@ -176,6 +176,19 @@ impl Model {
             .collect()
     }
 
+    /// The mesh nodes with rotational degrees of freedom under the model's own sections: those of
+    /// shells and beams, not of plane stress, plane strain or axisymmetric elements.
+    pub fn rotating_nodes(&self) -> BTreeSet<u32> {
+        let planar: BTreeSet<u32> = self
+            .sections
+            .values()
+            .filter(|s| s.kind.extent().is_some())
+            .filter_map(|s| self.resolve_set_partial(&s.set))
+            .flat_map(|r| r.elements)
+            .collect();
+        self.rotational_nodes(|l| planar.contains(&l))
+    }
+
     fn analyze(&self) -> (Resolved, Vec<Diagnostic>) {
         let mut d = Diagnostics::default();
         let mut resolved = Resolved::default();