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/engine/src/split_face_tests.rs

@@ -80,3 +80,20 @@ fn export_keeps_split_faces_through_later_features() {
     // Six box faces, the top's second piece and the fillet.
     assert_eq!(text.matches("ADVANCED_FACE(").count(), 8, "{text}");
 }
+
+#[test]
+fn split_pieces_count_and_move_apart() {
+    let mut s = boxed();
+    run(&mut s, "solid.split_face", json!({"faces": [[10, 15, 20]], "plane": {"origin": [20, 0, 0], "normal": [1, 0, 0]}}));
+    // Measure counts the two halves of the top as two faces.
+    let m = run(&mut s, "inspect.measure", json!({ "bodies": ["B"] }))["bodies"][0].clone();
+    assert_eq!(m["faces"].as_u64(), Some(7), "{m}");
+    // Offset Face on one half moves that half only (Fusion): a 20 × 30 step 5 high.
+    run(&mut s, "solid.offset_face", json!({"faces": [[10, 15, 20]], "distance": 5}));
+    let (_, v) = faces_volume(&mut s, "B");
+    assert!((v - 27000.0).abs() < 1e-6 * 27000.0, "{v}");
+    // And the other half down 3: a 20 × 30 pocket.
+    run(&mut s, "solid.offset_face", json!({"faces": [[30, 15, 20]], "distance": -3}));
+    let (_, v) = faces_volume(&mut s, "B");
+    assert!((v - 25200.0).abs() < 1e-6 * 25200.0, "{v}");
+}