feat(kernel): bodies joined apart report their history (#136)

union_apart joins bodies whose boxes are clearly apart (many pattern tools,
#261) without any intersection work: their shells side by side. Each face
and edge of the result is its piece's own, unchanged, and the result now
says so (a history over the pieces), so a pattern's cut keeps the exact
names of every copy's faces.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J53sR6gEaVt56soFDpTck8

crates/kernel/src/ops.rs

@@ -672,7 +672,22 @@ pub fn union_apart(bodies: &[Body]) -> Result<Option<Body>> {
             }
         }
     }
-    join_pieces(bodies.to_vec())
+    // The pieces stay as they are, side by side: each face and edge of the result is its piece's
+    // own (the history over `bodies`).
+    let Some(joined) = join_pieces(bodies.to_vec())? else { return Ok(None) };
+    let operands: Vec<&Body> = bodies.iter().collect();
+    let faces = bodies
+        .iter()
+        .enumerate()
+        .flat_map(|(operand, b)| {
+            (0..b.face_count()).map(move |face| (std::collections::BTreeSet::from([crate::history::FaceOrigin::Face { operand, face }]), true))
+        })
+        .collect();
+    let mut edges = HashMap::new();
+    for (operand, b) in bodies.iter().enumerate() {
+        edges.extend(record::edge_origins(&b.solid, operand));
+    }
+    Ok(Some(record::traced(joined, &record::counts(&operands), Some(record::regions(faces)), Some(&edges))))
 }
 
 /// Bodies (apart from each other) as one body of several pieces.