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/doc/src/eval.rs

@@ -283,6 +283,12 @@ impl ModelState {
                     (key, &b.name).hash(&mut h);
                     b.key = h.finish();
                     b.names = crate::naming::NameCell::new(Some(origin.clone()));
+                    // Faces split on purpose stay split through later features (for export).
+                    if b.body.split_lines().is_none()
+                        && let Some(lines) = before.bodies.iter().find(|x| x.name == b.name).and_then(|x| x.body.split_lines())
+                    {
+                        b.body = b.body.clone().with_split_lines(Some(lines.clone()));
+                    }
                     // An imported look outlives the operations on the body.
                     if b.body.paint().is_none()
                         && b.body.color().is_none()