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

docs/features.md

@@ -28,7 +28,7 @@ the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 | Size | Global Size — One element size for the whole part | coming soon (M2) |  |
 | Size | Seed Edges — Element counts, sizes and bias per edge and face | coming soon (M2) |  |
 | Size | Curvature — Refine by curvature and proximity | coming soon (M2) |  |
-| Tet | Tet Mesh — Constrained Delaunay tet mesh of a body: every boundary triangle kept, faces tagged (refinement comes next) | ready | `mesh.tet.generate` |
+| Tet | Tet Mesh — Quality tet4 or tet10 mesh of a body to a size: refined, smoothed and improved, boundary on the B-rep faces, faces tagged | ready | `mesh.tet.generate` |
 | Tet | Quadratic — Second-order elements with mid-side nodes on the geometry | coming soon (M2) |  |
 | Tet | Delaunay Points — Delaunay tetrahedralization of a point list (debugging aid) | ready | `mesh.delaunay.points` |
 | 2D | 2D Mesh — Quad or triangle mesh of a planar face or polygon in the XY plane for plane stress, plane strain and axisymmetric models; edges tagged | ready | `mesh.planar` |