Tet mesh quality: one validity criterion, Delaunay refinement, ODT, Klingner–Shewchuk, tet10

The end-to-end demo met a raw-CDT sliver that the validity gate accepted (exactly positive) and
the solver refused. Now:

- One validity criterion, `fieldcraft_model::admissible_jacobian` (normalized Jacobian above
  1e-12), used by the implicit solver (fem `positive_inverse`), the explicit tet4/hex8
  kinematics and the mesh gate, which proves it on the whole element (tet4: the solvers' own
  Jacobian; curved kinds: a Bernstein floor), so whatever passes the gate passes the solvers.
- CDT rounding repaired by flips on the rounded mesh (Diazzi et al. 2023 §5.1) instead of the
  ±2-ulp nudge.
- `fieldcraft-mesh::tet`: a mutable tetrahedral complex whose every change is a checked
  replacement (positive and admissible tetrahedra, faces matched with opposite orientation) with
  an exact undo log; edge removal (Klincsek), multi-face removal, Delaunay insertion with
  boundary faces, edge collapse; a curvature- and thickness-graded size field; constrained
  Delaunay refinement with encroachment splits; ODT smoothing; the Klingner–Shewchuk schedule;
  boundary vertices moved onto the exact B-rep surfaces at the end; tet10 with mid-edge nodes on
  the geometry.
- `mesh.tet.generate` gains size, order, quality, quality_target, curvature_angle, grading,
  max_passes, max_points and reports the CDT's and the mesh's quality; `solve.probe` takes a set
  and reports its largest von Mises stress.
- Oracle cases filleted-bracket-fillet-stress(-coarse): the fillet's peak von Mises stress on
  tet10 meshes of 2k and 13.5k elements, 213.3 MPa (a four-level study stays within 1.8%); the
  CDT cases keep the raw tetrahedralization.
- Corpus quality columns: all 20 valid parts mesh and pass the gate.

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

crates/fem/src/element.rs

@@ -35,7 +35,7 @@
 use fieldcraft_model::ElementKind;
 
 use crate::basis::Basis;
-use crate::dense::{Cholesky, Matrix, inverse3, positive_inverse};
+use crate::dense::{Cholesky, Matrix, positive_inverse};
 use crate::dof::{DofSet, ElementDofs, Internal};
 use crate::material::{Elasticity, stress};
 use crate::quadrature::{Point, Rule};
@@ -356,7 +356,7 @@ impl<'a> Solid<'a> {
 
     fn centre_jacobian(&self) -> Result<([[f64; 3]; 3], f64)> {
         let s = (self.ty.shape)([0.0; 3]);
-        inverse3(&self.jacobian(&s)).filter(|(_, d)| *d > 0.0).ok_or_else(|| self.degenerate())
+        positive_inverse(&self.jacobian(&s)).ok_or_else(|| self.degenerate())
     }
 
     /// The parts of the stiffness with the incompatible modes: K_uu (when `with_uu`), K_ua