solver: ties by dual mortar constraints

Each secondary node follows the main surface under it as the weighted average
u_A = sum_J (M_AJ / sum_K M_AK) u_J with M_AJ = int psi_A N_J over the reference overlaps and
psi the secondary faces' dual shape functions (Wohlmuth 2000), so the secondary mortar
matrix is diagonal; the rows join the couplings' constraint elimination in every implicit
analysis. Uncovered secondary nodes stay free with a warning; heat transfer and explicit
dynamics refuse ties by name. Tested by the tie patch test on non-matching hex8/tet4 meshes.

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

crates/explicit/src/system.rs

@@ -215,6 +215,9 @@ impl System {
         if let Some(name) = model.couplings().keys().next() {
             return Err(Error::NotSupported(format!("couplings (coupling `{name}`) in explicit dynamics")));
         }
+        if let Some(name) = model.ties().keys().next() {
+            return Err(Error::NotSupported(format!("ties (tie `{name}`) in explicit dynamics")));
+        }
 
         // Elements by label, with their section's material.
         let mut materials: Vec<Material> = Vec::new();