Mesh: swept cross-sections cut into blocks automatically

A region of an extrusion's (or revolution's) cross-section that no
structured scheme fits is now cut by straight lines into blocks that fit,
before interval assignment, so the cuts are curves of the arrangement and
their counts are assigned with the rest; nothing reaches the model
(hex::decompose, after Tam and Armstrong's medial-axis subdivision and Li,
Armstrong and McKeag's primitives).

Per region: several holes are separated by lines across the gaps between
neighbouring holes; a piece with one hole gets the O-grid layout whose
pieces score best (lines from the outer corners towards the hole or along
their bisectors, from the hole's corners, or along four directions from
the hole); pieces without holes are cut by a two-ply search over lines
from concave corners and fillets and from convex corners. Candidates are
scored by meshing the pieces with their schemes (smallest corner scaled
Jacobian). A region's blocks are kept when fair (at least 0.5) or no worse
than its free mesh, and cuts never end inside a seeded curve.

This replaces the cuts from reflex corners along an edge. Tests on an L,
a T, a plate with a hole, with a slot and with three holes, and the half
crank's cross-section: every region structured, the quads conforming and
the smallest corner scaled Jacobian bounded. The half crank arm now
extrudes in 12 structured regions with a hex20 minimum scaled Jacobian of
0.37 (the arm meeting the bosses 20 degrees off tangent bounds it) where
it had 2 free regions and 0.009.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

docs/architecture.md

@@ -95,8 +95,10 @@ Rules beyond the layer order:
    (`fieldcraft-mesh::size`) sets interior sizes and edge divisions, with even counts per loop.
    Mapped and swept hex build on these face meshes (`fieldcraft-mesh::hex`, `mesh.hex.generate`):
    the B-rep topology is recovered from the face-tagged surface; a body is extruded (straight
-   multi-level sweep: the levels' outlines imprinted into one cross-section, cut at reflex corners,
-   regions mapped, O-gridded or free-meshed, then stacked), revolved (the same in a rotating
+   multi-level sweep: the levels' outlines imprinted into one cross-section, cut automatically
+   into blocks (holes apart, O-grids, cuts from concave corners and fillets chosen by the quality
+   of the blocks they leave), regions mapped, triangle primitives, O-gridded or free-meshed, then
+   stacked), revolved (the same in a rotating
    frame), swept one-to-one through rings of linking faces (least-squares affine layers, Roca and
    Sarrate 2010), or mapped as a logical cube (Gordon and Thiel 1982); interval assignment
    couples the counts of shared curves. Nodes on analytic faces (`Shape::face_surfaces`) are

docs/meshing/literature.md

@@ -1001,8 +1001,8 @@ record or the publisher; the others are cited by venue only.
 - **Citation.** M. Whiteley, D. White, S. Benzley, T. Blacker. *Two and three-quarter dimensional meshing
   facilitators.* Engineering with Computers 12:144–154, 1996.
 - **Use.** Corners by interior angle (≤ 135° end, ≥ 225° reflex); cross-section regions are cut from
-  their reflex corners to the boundary, which decomposes rectilinear sections into rectangles (an explicit
-  form of submapping's virtual decomposition), then mapped.
+  their reflex corners to the boundary (B8.6b), which decomposes rectilinear sections into rectangles (an
+  explicit form of submapping's virtual decomposition), then mapped.
 
 **B8.6a Midpoint subdivision: Tam and Armstrong 1991; Li, Armstrong, McKeag 1997.** **Verdict: BUILD ON
 (triangle primitive).**
@@ -1017,6 +1017,24 @@ record or the publisher; the others are cited by venue only.
   shorter than the other two together). The centre is the mean of the split nodes; the interior is then
   smoothed (`quad::smooth`) and every quadrilateral checked; a fold falls back to the free mesher.
 
+**B8.6b Automatic cross-section decomposition (medial-axis subdivision).** **Verdict: BUILD ON (blocks
+of 2.5D bodies).**
+- **Citations.** Tam and Armstrong 1991 (B8.6a): a region is subdivided at its concave features and the
+  branches of its medial axis into simple primitives. Li, Armstrong, McKeag 1997 (B8.6a): the primitives.
+  Whiteley et al. 1996 (B8.6): vertex classification.
+- **Use.** `hex::decompose` cuts the regions of a multi-level sweep's cross-section that no structured
+  scheme fits into blocks, before interval assignment (B8.5), so the cuts are curves of the arrangement
+  and their counts are assigned with the rest; nothing reaches the model. Per region: several holes are
+  separated by lines across the gaps between neighbouring holes; a piece with one hole gets an O-grid,
+  the layout (lines from the outer corners towards the hole or along their bisectors, from the hole's
+  corners, or along four directions from the hole) chosen by the score of its pieces; pieces without
+  holes are cut by a two-ply search over lines from concave corners and fillets (bisector, edge
+  extensions, normals of strongly turning curves) and convex corners' bisectors. In place of the
+  medial axis's geometric rules each candidate is scored by meshing its pieces with their schemes (the
+  smallest corner scaled Jacobian), preferring fewer free pieces, then less shortfall from 0.9, then
+  fewer pieces. A region's blocks are kept when fair (≥ 0.5) or no worse than its free mesh; cuts never
+  end inside a seeded curve.
+
 **B8.7 Gordon and Thiel 1982, trivariate transfinite interpolation.** **Verdict: BUILD ON (mapped blocks).**
 - **Citation.** W. J. Gordon, L. C. Thiel. *Transfinite mappings and their application to grid generation.*
   Applied Mathematics and Computation 10–11:171–233, 1982.