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/vec3.rs

@@ -23,3 +23,18 @@ pub(crate) fn det(a: P, b: P, c: P) -> f64 {
 pub(crate) fn angle_deg(a: P, b: P) -> f64 {
     norm(cross(a, b)).atan2(dot(a, b)).to_degrees()
 }
+pub(crate) fn add(a: P, b: P) -> P {
+    [a[0] + b[0], a[1] + b[1], a[2] + b[2]]
+}
+pub(crate) fn scale(a: P, s: f64) -> P {
+    [a[0] * s, a[1] * s, a[2] * s]
+}
+pub(crate) fn dist(a: P, b: P) -> f64 {
+    norm(sub(a, b))
+}
+pub(crate) fn lerp(a: P, b: P, t: f64) -> P {
+    [a[0] + (b[0] - a[0]) * t, a[1] + (b[1] - a[1]) * t, a[2] + (b[2] - a[2]) * t]
+}
+pub(crate) fn mid(a: P, b: P) -> P {
+    [0.5 * (a[0] + b[0]), 0.5 * (a[1] + b[1]), 0.5 * (a[2] + b[2])]
+}