fix(doc): designs saved before format 14 keep their piece names (#136)

Pieces a cut splits a body into are named after their faces since names
follow history (lineage::order_lumps), so they keep their body names when
an upstream edit moves the cut. The kernel's shell order named them
before, and the two orders can differ: a slot along X through a 40x30x10
block swaps Body1 and Body1 (1). A design saved by an older build stores
body names meaning the old order (fillet and chamfer bodies, targets,
participants, materials, appearances, joints), so it would act on the
other piece.

Each feature now says which order names its pieces
(Feature::shell_piece_order). The v13 to v14 step sets it on every saved
feature, so an older design evaluates as it was saved; features made from
now on, in new or old designs, name their pieces after their faces.

The corpus gains split-body-pieces, saved by v0.3.2: a fillet, a chamfer
and an appearance refer to the two pieces by body name. Without the
setting the two pieces swap names (the corpus test fails); with it they
stay as saved. Migration tests that compared an upgraded design with the
original now expect the setting on its features.

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

crates/doc/src/document.rs

@@ -1567,6 +1567,11 @@ pub struct Feature {
     /// component's bodies.
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub participants: Vec<String>,
+    /// The pieces this feature splits a body into take its body names in the kernel's shell order,
+    /// as builds before format 14 named them, instead of after their faces (`lineage::order_lumps`).
+    /// Set on the features of designs saved before then, whose stored body names mean that order.
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub shell_piece_order: bool,
     #[serde(flatten)]
     pub kind: FeatureKind,
 }
@@ -1901,6 +1906,7 @@ impl Document {
             edge_names: Vec::new(),
             face_names: Vec::new(),
             participants: Vec::new(),
+            shell_piece_order: false,
             kind,
         };
         match self.marker {