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/geometry/src/lib.rs

@@ -47,6 +47,9 @@ impl From<KernelError> for Error {
     }
 }
 
+/// SolidCraft's linear modelling tolerance (mm): points closer than this are the same point.
+const LINEAR_TOLERANCE: f64 = solidcraft_geom::LINEAR_TOL;
+
 /// A solid body.
 #[derive(Debug, Clone)]
 pub struct Shape {
@@ -86,6 +89,11 @@ pub struct SurfaceMesh {
     pub triangles: Vec<[u32; 3]>,
     /// The B-rep face each triangle lies on.
     pub tri_face: Vec<u32>,
+    /// Copies of one point closer than this are the same point (0: only identical coordinates).
+    /// A [`Shape::tessellate`] result is a triangle soup whose faces each carry their own copy of
+    /// the boundary points they share, accurate to the kernel's linear tolerance, so two copies
+    /// can differ by up to twice it; boundary surface preparation welds at this distance.
+    pub weld_distance: f64,
 }
 
 fn xyz(v: solidcraft_geom::Vec3) -> [f64; 3] {
@@ -129,7 +137,12 @@ impl Shape {
             return Err(Error::Invalid(format!("tessellation tolerance must be a positive number, got {tolerance}")));
         }
         let m = guard("tessellate", || self.body.tessellate(tolerance))?;
-        Ok(SurfaceMesh { positions: m.positions.into_iter().map(xyz).collect(), triangles: m.triangles, tri_face: m.tri_face })
+        Ok(SurfaceMesh {
+            positions: m.positions.into_iter().map(xyz).collect(),
+            triangles: m.triangles,
+            tri_face: m.tri_face,
+            weld_distance: 2.0 * LINEAR_TOLERANCE,
+        })
     }
 }
 

crates/geometry/src/tagged_mesh.rs

@@ -341,7 +341,7 @@ fn surface(mut o: Object, k: usize, id: u32, xf: &Xf, unit: f64) -> Result<Tagge
     }
     let faces = o.faces.unwrap_or_else(|| o.tags.iter().max().map_or(1, |f| f.saturating_add(1)));
     let name = if o.name.is_empty() { format!("Mesh{}", k + 1) } else { o.name };
-    Ok(TaggedSurface { name, mesh: SurfaceMesh { positions: o.positions, triangles: o.triangles, tri_face: o.tags }, faces })
+    Ok(TaggedSurface { name, mesh: SurfaceMesh { positions: o.positions, triangles: o.triangles, tri_face: o.tags, weld_distance: 0.0 }, faces })
 }
 
 /// [`read_3mf`] of a file: a regular file of at most [`MAX_FILE_BYTES`] (SolidCraft's virtual
@@ -482,6 +482,8 @@ mod tests {
         assert_eq!(bytes, write_tagged(&[("Block", &mesh, faces), ("Again", &mesh, faces)]), "deterministic bytes");
         let got = read_3mf(&bytes).unwrap();
         assert_eq!(got.len(), 2);
+        // The file carries no weld distance.
+        let mesh = SurfaceMesh { weld_distance: 0.0, ..mesh };
         assert_eq!(got[0], TaggedSurface { name: "Block".into(), mesh: mesh.clone(), faces: 6 });
         assert_eq!(got[1].name, "Again");
         assert_eq!(got[1].mesh, mesh);
@@ -577,7 +579,11 @@ mod tests {
         }
         assert!(read_3mf(b"").is_err());
         assert!(read_3mf(b"PK\x03\x04garbage").is_err());
-        let good = write_tagged(&[("T", &SurfaceMesh { positions: vec![[0.0; 3]; 3], triangles: vec![[0, 1, 2]], tri_face: vec![0] }, 1)]);
+        let good = write_tagged(&[(
+            "T",
+            &SurfaceMesh { positions: vec![[0.0; 3]; 3], triangles: vec![[0, 1, 2]], tri_face: vec![0], weld_distance: 0.0 },
+            1,
+        )]);
         for cut in [10, good.len() / 2, good.len() - 1] {
             assert!(read_3mf(&good[..cut]).is_err(), "truncated at {cut}");
         }