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 <noreply@anthropic.com>
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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: struct-init the test session

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

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/geometry.rs

@@ -1,7 +1,7 @@
 use std::sync::Arc;
 
 use modelcraft_geometry::tagged_mesh;
-use modelcraft_geometry::{Shape, open_design_file};
+use modelcraft_geometry::{Shape, SurfaceMesh, open_design_file};
 use serde_json::{Value, json};
 
 use crate::{CommandSpec, EngineError, Result, Session};
@@ -72,10 +72,15 @@ fn body<'a>(s: &'a Session, p: &Value) -> Result<&'a Shape> {
     s.design.as_deref().and_then(|d| d.body(name)).ok_or_else(|| EngineError::Params(format!("no body `{name}` (open a design first)")))
 }
 
-fn tessellate(s: &mut Session, p: &Value) -> Result<Value> {
+/// The body named by `body`, tessellated within `tolerance`.
+pub(crate) fn body_mesh<'a>(s: &'a Session, p: &Value) -> Result<(&'a Shape, SurfaceMesh)> {
     let shape = body(s, p)?;
     let tolerance = p.get("tolerance").and_then(Value::as_f64).ok_or_else(|| EngineError::Params("`tolerance` must be a number (mm)".into()))?;
-    let mesh = shape.tessellate(tolerance)?;
+    Ok((shape, shape.tessellate(tolerance)?))
+}
+
+fn tessellate(s: &mut Session, p: &Value) -> Result<Value> {
+    let (shape, mesh) = body_mesh(s, p)?;
     let mut per_face = vec![0usize; shape.faces().len()];
     for &f in &mesh.tri_face {
         if let Some(n) = per_face.get_mut(f as usize) {