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