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_corner.rs

@@ -33,7 +33,7 @@ pub fn fillet_radii(body: &Body, edges: &[(Vec3, f64)]) -> Result<Body> {
     if edges.iter().any(|(p, r)| !(p.is_finite() && r.is_finite() && *r > 1e-6 && *r < 1e6)) {
         return Err(KernelError::Invalid("fillet size must be positive".into()));
     }
-    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()))?;
     let mut sel = edges.to_vec();
     let mut first: Option<KernelError> = None;
     // Earlier rounds at the edges' ends come off and are blended again with the new edges.