Split pieces stay separate faces: Measure counts them apart and Offset Face moves only the picked piece

The merged face count no longer joins faces across a Split Face line, and Offset Face's same-surface grouping stops there. A piece whose move the body's topology cannot follow (its neighbour stays put) is swept along its normal and joined on or cut in, giving walls along the split line as in Fusion. Engine test: a split box top counts 7 faces; one half up 5 (27000 mm³), the other down 3 (25200 mm³).

Fixes #12

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

crates/kernel/src/polyhedron.rs

@@ -422,7 +422,7 @@ pub fn offset_faces(b: &Body, at: &[Vec3], distance: f64) -> Result<Body> {
     if !distance.is_finite() || distance.abs() > 1e6 {
         return Err(KernelError::Invalid("offset distance".into()));
     }
-    let healed = Body::new(crate::heal::heal_keep(b.deep_copy(), b.size(), b.split_keep()))?;
+    let healed = Body::new(crate::heal::heal_keep(b.deep_copy(), b.size(), b.split_keep()))?.with_split_lines(b.splits.clone());
     let size = healed.size();
     let mesh = healed.tessellate((size * 1e-3).max(1e-3))?;
     let mut chosen: Vec<usize> = Vec::new();
@@ -443,5 +443,24 @@ pub fn offset_faces(b: &Body, at: &[Vec3], distance: f64) -> Result<Body> {
         }
     }
     let chosen = crate::offset::with_same_surface(&healed, &chosen);
-    offset_planar(&healed, |fi, _| if chosen.contains(&fi) { distance } else { 0.0 })
+    match offset_planar(&healed, |fi, _| if chosen.contains(&fi) { distance } else { 0.0 }) {
+        Ok(r) => Ok(r),
+        // A piece of a split face (its neighbour stays): the piece swept along its normal
+        // joins on (outward) or cuts in, with walls along the split lines.
+        Err(e) if !healed.split_keep().is_empty() => press_pieces(&healed, &chosen, distance).map_err(|_| e),
+        Err(e) => Err(e),
+    }
+}
+
+fn press_pieces(b: &Body, chosen: &[usize], distance: f64) -> Result<Body> {
+    let faces: Vec<mt::Face> = b.solid.face_iter().cloned().collect();
+    let mut cur = b.clone();
+    for fi in chosen {
+        let f = faces.get(*fi).ok_or_else(|| KernelError::Failed("offset: face".into()))?;
+        let n = crate::blend::plane_normal(f).ok_or_else(|| KernelError::Failed("offset: only planar pieces".into()))?;
+        let tool = guard("offset face", || Body::new(builder::tsweep(f, crate::body::v3(n * distance))))?;
+        let op = if distance > 0.0 { crate::BoolOp::Union } else { crate::BoolOp::Cut };
+        cur = crate::ops::boolean(&cur, &tool, op)?.ok_or_else(|| KernelError::Failed("offset: the piece moved off the body".into()))?;
+    }
+    Ok(cur.with_split_lines(b.splits.clone()))
 }