Implicit solver: reference points, couplings, moments, wedge6 (M4) (#26)

* fem: wedge6 element (triangle × interval, 3 × 2 points), patch and cantilever tests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* solver: reference points, kinematic and distributing couplings, moments by constraint elimination

Coupled reference points get six degrees of freedom; kinematic couplings are rigid
links, distributing couplings least-squares interpolation constraints. Every
constraint row and prescribed value is eliminated master-slave (Abel-Shephard
pivot choice), so the reduced matrix stays SPD; no coupling gives the old
elimination bit for bit. Verified: rigid motion of a coupled block, clamping
through a point, distributing tension vs pressure, bending and tube torsion.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: reference point results in solve.static and solve.probe

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: couplings, moments and wedge6 in the solver docs, literature and roadmap

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* solver: simplify constraint elimination and reference point plumbing

Rows combine by one sort; T_f transposed once; K and M reduced in one pass on a
shared pattern; prescribed motion computed once; reference point loads and BCs go
through Resolved like node ones (Mesh::label_dofs); one dof-direction rule;
StepResult::locate for probes; dead Csr::restrict removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* solver: refuse oversized reductions before allocating them

A held distributing coupling point makes a dense (3N)^2 block; estimate it from
the constraint rows and refuse it against the factorization's memory budget
with a clear message, before anything is built. Other reductions with
dependents count their entries first (stopping at the budget) and reserve
fallibly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/solvers/literature.md

@@ -977,6 +977,17 @@ the module docs):
   Lewis, H. D. Simon, SIAM J. Matrix Anal. Appl. 15(1):228–272, 1994; B. N. Parlett, *The
   Symmetric Eigenvalue Problem*, SIAM 1998; G. H. Golub, C. F. Van Loan, *Matrix Computations*,
   4th ed., 2013.
+- Wedge6: the linear triangle times the linear interval; the symmetric three-point triangle rule
+  (Zienkiewicz, Taylor & Zhu 2013, table 6.3) times two-point Gauss–Legendre.
+- Couplings and constraint elimination: rigid links and master–slave elimination by
+  transformation, R. D. Cook, D. S. Malkus, M. E. Plesha, R. J. Witt, *Concepts and Applications
+  of Finite Element Analysis*, 4th ed., Wiley 2002, §9.2, and C. A. Felippa, *Introduction to
+  Finite Element Methods*, ch. 8 (University of Colorado lecture notes); automatic choice of the
+  dependent degree of freedom, J. F. Abel, M. S. Shephard, IJNME 14(3):464–467, 1979; the
+  distributing (interpolation) constraint as a least-squares rigid fit with loads spread by
+  virtual work, G. H. K. Heirman, W. Desmet, Multibody Syst. Dyn. 24(2):219–234, 2010; the
+  reduced matrix product T_fᵀ K T_f row by row, F. G. Gustavson, ACM TOMS 4(3):250–269, 1978.
 - Verification: R. H. MacNeal, R. L. Harder, *A proposed standard set of problems to test finite
   element accuracy*, Finite Elem. Anal. Des. 1(1):3–20, 1985 (patch tests); Lamé, Kirsch,
-  Timoshenko and Euler–Bernoulli closed forms.
+  Timoshenko and Euler–Bernoulli closed forms; Saint-Venant's principle (tension through a
+  coupling), pure bending σ = M z / I, torsion of a circular tube φ = T L / (G J), τ = T r / J.