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/lib.rs

@@ -40,6 +40,7 @@ pub mod nonlinear;
 pub mod ordering;
 pub mod sparse;
 pub mod system;
+pub mod tie;
 
 pub use analysis::{
     ElementStress, FrequencyOptions, FrequencyResult, Location, MassKind, Mode, PointResult, StaticResult, Stats, StepResult, solve_frequency,