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/mesh/src/cdt/tests.rs

@@ -10,7 +10,7 @@ use crate::vec3::{cross, dot, norm, sub};
 
 /// A closed surface from points and triangles, each triangle oriented outward from `centre`
 /// (the shapes here are star-shaped about it).
-fn surface(points: &[[f64; 3]], triangles: &[[u32; 3]], centre: [f64; 3]) -> BoundarySurface {
+pub(crate) fn surface(points: &[[f64; 3]], triangles: &[[u32; 3]], centre: [f64; 3]) -> BoundarySurface {
     let oriented: Vec<[u32; 3]> = triangles
         .iter()
         .map(|&[a, b, c]| {
@@ -47,7 +47,7 @@ fn check(s: &BoundarySurface, m: &TetMesh) {
     }
 }
 
-fn cube() -> (Vec<[f64; 3]>, Vec<[u32; 3]>) {
+pub(crate) fn cube() -> (Vec<[f64; 3]>, Vec<[u32; 3]>) {
     let p: Vec<[f64; 3]> = (0..8).map(|i| [(i & 1) as f64, (i >> 1 & 1) as f64, (i >> 2 & 1) as f64]).collect();
     let quads = [[0, 1, 3, 2], [4, 5, 7, 6], [0, 1, 5, 4], [2, 3, 7, 6], [0, 2, 6, 4], [1, 3, 7, 5]];
     let t = quads.iter().flat_map(|q| [[q[0], q[1], q[2]], [q[0], q[2], q[3]]]).collect();
@@ -98,7 +98,7 @@ fn schonhardt_prism_gets_steiner_points_and_conforms() {
 }
 
 /// A UV sphere: many cospherical vertices.
-fn uv_sphere(rings: usize, sectors: usize) -> (Vec<[f64; 3]>, Vec<[u32; 3]>) {
+pub(crate) fn uv_sphere(rings: usize, sectors: usize) -> (Vec<[f64; 3]>, Vec<[u32; 3]>) {
     let mut p = vec![[0.0, 0.0, 1.0], [0.0, 0.0, -1.0]];
     for r in 1..rings {
         let th = std::f64::consts::PI * r as f64 / rings as f64;