Faces split on purpose stay split: the body remembers its Split Face lines, export and healing keep the pieces apart

Split Face records the new edges as polylines on the body (Body::split_lines); the document carries them through later features. STEP export no longer joins pieces of one surface across such a line, and healing (fillets, chamfers, booleans, shell, draft) no longer merges coplanar pieces there, while incidental pieces (boolean leftovers, seam halves) still merge as before. Tests: plane and cylinder splits keep their face count through a STEP round trip (and join without the record), a second split keeps both lines, and an engine test exports a split box after a fillet with 8 faces.

Fixes #11

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

crates/kernel/src/meshbody.rs

@@ -101,7 +101,7 @@ pub fn mesh_body(positions: &[Vec3], triangles: &[[u32; 3]]) -> Result<Body> {
             t.swap(1, 2);
         }
     }
-    Ok(Body { solid: Arc::new(crate::body::Solid::new_unchecked(Vec::new())), mesh: Some(Arc::new(out)), color: None, paint: None })
+    Ok(Body { solid: Arc::new(crate::body::Solid::new_unchecked(Vec::new())), mesh: Some(Arc::new(out)), color: None, paint: None, splits: None })
 }
 
 /// Wind every connected piece one way: a triangle that runs a side in the same direction as
@@ -185,6 +185,6 @@ impl Body {
                 t.swap(1, 2);
             }
         }
-        Some(Body { solid: self.solid.clone(), mesh: Some(Arc::new(out)), color: self.color, paint: self.paint.clone() })
+        Some(Body { solid: self.solid.clone(), mesh: Some(Arc::new(out)), color: self.color, paint: self.paint.clone(), splits: self.splits.clone() })
     }
 }