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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

ROADMAP.md

@@ -28,7 +28,7 @@ pressure) is a degree of freedom of the existing DOF framework, not a separate s
 |---|---|---|
 | M0 Foundation | Workspace, quality gates, FE model, command engine with rollback and panic guard, CLI, geometry boundary on SolidCraft | done |
 | M1 Geometry in and surface preparation | Open `.solidcraft` designs with persistent face names, assemblies placed (done); face-tagged 3MF (done); test corpus, `cargo xtask corpus` (done); exact predicates (done); quality metrics and certified validity gate (done); weld, closed/manifold/orientation and self-intersection checks, feature edges (`fieldcraft-mesh::surface`, `mesh.surface.check`, done) | in progress |
-| M2 Tet meshing and size field | Size field (global, per face and edge seeds, curvature and proximity; the `fieldcraft_mesh::size::SizeField` trait and edge-division helpers are in); Bowyer–Watson Delaunay (exact, symbolic perturbation, verified; `fieldcraft-mesh::delaunay`, done); robust CDT boundary recovery (Diazzi et al. 2023: implicit LNC Steiner points on segments, gift-wrapping facet recovery, inside/outside labelling, B-rep face tags on boundary tet faces, few-ulp rounding repair checked by the validity gate; `fieldcraft-mesh::cdt` and `mesh.tet.generate`, done: all 20 valid corpus parts); Delaunay refinement, ODT smoothing, sliver removal; tet4 and tet10 with mid-side nodes on the B-rep; mesh verify per set | in progress |
+| M2 Tet meshing and size field | Size field (global, per face and edge seeds, curvature and proximity; the `fieldcraft_mesh::size::SizeField` trait and edge-division helpers are in); Bowyer–Watson Delaunay (exact, symbolic perturbation, verified; `fieldcraft-mesh::delaunay`, done); robust CDT boundary recovery (Diazzi et al. 2023: implicit LNC Steiner points on segments, gift-wrapping facet recovery, inside/outside labelling, B-rep face tags on boundary tet faces, flip repair of rounding (§5.1) checked by the validity gate; `fieldcraft-mesh::cdt` and `mesh.tet.generate`, done: all 20 valid corpus parts); quality stage (done: `fieldcraft-mesh::tet` — curvature-graded size field, edge-collapse coarsening, constrained Delaunay refinement with boundary points on the B-rep faces and curves, ODT smoothing, Klingner–Shewchuk improvement, tet4 and tet10 with mid-side nodes on the B-rep, quality report; one validity criterion shared by the gate and both solvers; the filleted-bracket fillet stress converges, tests/oracle); mesh verify per set | in progress |
 | M3 First deck | Sets and surfaces bound to SolidCraft face, edge, vertex and body names, resolved after meshing; materials (isotropic and orthotropic elastic, density) with a unit-system label and consistency check; sections (solid, shell, beam profiles) assigned by set; element formulation per region mapped to each solver; BCs (fixed, displacement, symmetry); loads (force and moment on reference points, pressure, gravity); reference points with kinematic and distributing coupling; steps (linear static, frequency) with explicit per-step load and BC tables and output presets; `model.check` naming every missing or bad item; `model.*` and `mesh.tag` commands (model definition: done); writers for Abaqus input (done: `deck.write`), Nastran SOL 101/103 (done: `deck.write`) and LS-DYNA implicit (done); the `.fieldcraft` file (text definition, binary mesh, geometry references reopened on load, journal; atomic saves, autosave and recovery; undo/redo on the journal: done) | in progress |
 | M4 Own solver and first results | Element library per docs/elements.md (1D and 2D first); Linear static and modal solver (tet4/10, wedge6, hex8, hex20; sparse direct, Lanczos), verified against analytic solutions (solver: done — `fieldcraft-fem` kernels, `fieldcraft-solver` with nested dissection, sparse Cholesky and block Lanczos, `solve.static`/`solve.frequency`/`solve.probe`, patch, cantilever, Lamé, Kirsch and beam-frequency tests; reference points with kinematic and distributing couplings and moments by master–slave constraint elimination, verified by rigid motion, clamping, Saint-Venant tension, bending and tube torsion; 1D elements I: rod2/rod3 trusses, point masses, rotary inertia, springs and dashpots in both solvers and all three decks, verified by the method of joints, the three-bar truss, spring–mass, two-mass chain and torsional pendulum frequencies, and damped decay; 2D elements: plane stress, plane strain and axisymmetric tri3, tri6, quad4 (incompatible modes QM6), quad8 with two degrees of freedom per node, centrifugal loads, `mesh.planar` 2D meshes of polygons and planar faces, verified by patch tests, Kirsch, Lamé (plane strain and axisymmetric), rotating discs and cylinders, and a plane stress cantilever; deck mapping to Abaqus CPS/CPE/CAX, Nastran membranes, LS-DYNA ELFORM 12/13); shells and beams (done: quad4 MITC4 and tri3 MITC3 flat shells with a Hughes–Brezzi drilling rotation, two-node Timoshenko beams with profile properties and orientations, six degrees of freedom at shell and beam nodes shared with solids and trusses, section forces and surface stresses, verified by shell patch tests, Navier's plate, cantilever and portal-frame beams, and the Scordelis–Lo and pinched-cylinder benchmarks); results display (done: `results.*` commands, deformed shape to scale, banded contours of displacement, von Mises, principal and component stresses with a legend, mode shapes with their frequencies and animation, probing a node by clicking it, reaction force and moment sums per set, stale results marked after an edit; explicit steps: field frames by slider or animation with contours per frame, energy histories plotted) | started |
 | M5 Structured hex | 2D quad meshing of faces (`fieldcraft-mesh::quad`: transfinite mapped meshes, free all-quad meshes of polygons with holes on prescribed boundary nodes, exact planar quad validity, shared `SizeField` and even edge divisions; done for planar faces); mapped hex of six-sided regions (trivariate TFI, curved faces) and sweep meshing (multi-level extrusions with imprinted cross-sections, revolutions, one-to-one sweeps through linking faces; interval assignment, hex8/hex20 on the geometry, B-rep face tags, `mesh.hex.generate`/`sweep`/`mapped`: done, 16 of 21 corpus parts all-hex); conforming interfaces between cells (done: `mesh.hex.blocks` meshes a part split with SolidCraft's Split Body into mapped blocks as one conforming mesh — composite faces for seams and tangent faces, interval assignment over all cells, shared faces meshed once, grading off holes and fillets, free-internal-face check; the filleted bracket in 30 cells, hex20 within 0.1 % of a fine tet10 tip deflection); sweepable cells in a conforming mesh and tet–hex transition; automatic decomposition into sweepable and mappable regions, proposed as SolidCraft splits | started |