mesh: unstructured all-hex octree mesher

`fieldcraft_mesh::octree::octree_hex_mesh` meshes any closed part with hex8 only:

- an adaptive 3-refinement grid (27-tree) with strong balance and closure, filled with
  conforming node-marking transition templates (Schneiders; checked over all 48 symmetries);
- a core of grid cells kept clear of the boundary (the HybridOctree buffer rule), with
  non-manifold cleanup and a genus check against the part;
- a feature graph from the B-rep (corners, curves, patches) and a partition-first map of the
  core's boundary onto it, then two boundary layers: a padding layer and a pillow per patch,
  so every face boundary and sharp edge is captured by element edges;
- a Garanzha-type untangling and fitting energy (shape term |J|^3/chi, a volume term and a
  soft fitting term, deterministic across platforms), a compass search on the worst
  elements and a certified repair; failures refine the grid near them and rebuild, within
  bounded attempts and growth.

The output passes the certified Jacobian check of every element and a gate that the
boundary encloses the elements' volume; boundary quads carry their B-rep faces. Quality is
reported with the same scaled Jacobian as mesh.quality. Literature notes for each step are in
docs/meshing/literature.md (B1.6-B1.10).

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

crates/mesh/src/hex/mod.rs

@@ -37,7 +37,7 @@
 mod arrangement;
 mod brep;
 mod cells;
-mod geom;
+pub(crate) mod geom;
 mod interval;
 mod layered;
 mod mapped;
@@ -583,7 +583,7 @@ fn finish(cells: &Cells, raw: Raw, method: HexMethod) -> Result<HexMesh> {
 
 /// `∫ x n_x dA` over a quad4 or quad8 face (outward normal): its contribution to the enclosed
 /// volume. The 3 × 3 Gauss rule integrates it exactly (degree at most 5 per direction).
-fn face_flux(face: &[P]) -> f64 {
+pub(crate) fn face_flux(face: &[P]) -> f64 {
     const G: [(f64, f64); 3] = [(-0.774_596_669_241_483_4, 5.0 / 9.0), (0.0, 8.0 / 9.0), (0.774_596_669_241_483_4, 5.0 / 9.0)];
     let mut sum = 0.0;
     for (r, wr) in G {