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/blend.rs

@@ -285,7 +285,7 @@ fn blend(body: &Body, edges: &[Vec3], r: f64, shape: Shape, what: &str) -> Resul
         return Err(KernelError::Invalid("too many edges".into()));
     }
     // Coplanar neighbours as one face (as after a join), so corners look as they should.
-    let mut cur = Body::new(crate::heal::heal(body.deep_copy(), body.size()))?;
+    let mut cur = Body::new(crate::heal::heal_keep(body.deep_copy(), body.size(), body.split_keep()))?;
     // Every edge of a convex planar body: rounded as a whole (sphere corners).
     if shape == Shape::Round && edges.len() >= 6 {
         let solid = cur.deep_copy();
@@ -700,7 +700,7 @@ pub fn chamfer(body: &Body, edges: &[Vec3], distance: f64) -> Result<Body> {
         }
     }
     // Coplanar neighbours as one face (as after a join), so corners look as they should.
-    let mut cur = Body::new(crate::heal::heal(body.deep_copy(), body.size()))?;
+    let mut cur = Body::new(crate::heal::heal_keep(body.deep_copy(), body.size(), body.split_keep()))?;
     if let Some(groups) = loop_groups(&cur, edges)? {
         for g in groups {
             let size = cur.size();