Boundary surface preparation (weld, manifold/orientation, exact self-intersection, feature edges) and mesh.surface.check (#15)

* mesh: boundary surface preparation (weld, manifold and orientation checks, exact self-intersection, feature edges)

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: mesh.surface.check on an open design's body or an imported surface

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: struct-init the test session

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* surface: weld at an absolute distance too; the engine welds CAD tessellations at twice SolidCraft's linear tolerance

Faces of a SolidCraft tessellation carry their own copies of shared boundary points, which agree
only to within the kernel's tolerance (up to ~2e-6 mm on the test corpus).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: surface preparation in the literature survey, architecture and roadmap

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* surface: simplify after review

- The weld distance travels with the mesh (SurfaceMesh::weld_distance, set by Shape::tessellate),
  so every consumer welds a tessellation correctly without opting in.
- Exact tests are infallible on predicates::orient2d_projected (shared with generic_orient2d), and
  compute each vertex's plane side once per pair with an early reject.
- One Box3 for the weld, the BVH and shells; a Half struct for edges; the weld's two lookups share
  one loop, with chained grid cells; cached normals for feature edges; shared parameter helpers.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* corpus: boundary surface preparation as a corpus stage

20 of 21 parts are valid boundaries with their expected genus; cross-cylinders' tessellation has
cracks and flipped triangles where its two intersection curves meet.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/meshing/literature.md

@@ -110,6 +110,23 @@ Delaunay methods with Voronoi/ODT improvement, then (2) all-hex and hex-dominant
 
 ---
 
+**A0.5 Boundary surface preparation (implemented in `modelcraft-mesh::surface`).** **Verdict: BUILD ON.**
+- **Guigue and Devillers 2003.** P. Guigue, O. Devillers. *Fast and Robust Triangle–Triangle Overlap Test Using
+  Orientation Predicates.* Journal of Graphics Tools 8(1):25–32, 2003. Triangle overlap decided from `orient3d` /
+  `orient2d` signs alone, so it is exact on the A0.1 predicates. We use the equivalent edge–triangle reduction (two
+  closed triangles meet exactly when an edge of one meets the other) and add the cases of triangles that share a
+  vertex or an edge, which may meet only there.
+- **Ericson 2005.** C. Ericson. *Real-Time Collision Detection.* Morgan Kaufmann, 2005, ch. 6. Bounding volume
+  hierarchies: top-down median-split construction and the self-collision traversal that pairs the triangles to test.
+- **Jacobson, Kavan, Sorkine-Hornung 2013.** *Robust Inside-Outside Segmentation using Generalized Winding Numbers.*
+  ACM TOG 32(4):33, 2013. DOI [10.1145/2461912.2461916](https://doi.org/10.1145/2461912.2461916). The winding number
+  of a vertex of one shell with respect to another tells nesting, so voids must face inwards.
+- **van Oosterom and Strackee 1983.** *The Solid Angle of a Plane Triangle.* IEEE Trans. Biomed. Eng. 30(2):125–126,
+  1983. DOI [10.1109/TBME.1983.325207](https://doi.org/10.1109/TBME.1983.325207). The triangle solid angle the
+  winding number sums.
+
+---
+
 ### A1. Delaunay construction
 
 **A1.1 Bowyer 1981 / Watson 1981, incremental cavity insertion.** **Verdict: build on.**