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/solver/src/analysis.rs

@@ -341,7 +341,8 @@ pub(crate) struct Setup<'a> {
 }
 
 pub(crate) fn reduction(s: &Setup, values: bool, order: &[usize], budget: Budget, warnings: &mut Vec<String>) -> Result<Reduction> {
-    let rows = coupling_rows(s.model, s.resolved, s.mesh)?;
+    let mut rows = coupling_rows(s.model, s.resolved, s.mesh)?;
+    rows.extend(crate::tie::rows(s.model, s.resolved, s.mesh, warnings)?);
     Reduction::new(s.mesh, order, &rows, |active| prescribed(s.model, s.resolved, s.mesh, s.step, values, active, warnings), budget)
 }