STEP assemblies: mesh a repeated part once and place it for its instances (#1152)

The reader converts each item once, but every instance made its own display mesh
(tessellation, seam and tangent flags), the most expensive step of opening a large
assembly. A later instance of an item now shows its first instance's mesh, placed by
the difference of the two placements. Only rigid placements share; split lines and
any operation drop the link. Measurements and export read each instance's own
geometry, so eval output is unchanged.

Generated assemblies: mesh phase 2,215 -> 139 ms (901 bodies) and 1,028 -> 85 ms
(5,401 bodies), with the same triangle counts.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/kernel/src/splitface.rs

@@ -558,6 +558,7 @@ pub(crate) fn split_edges_at(body: &Body, points: &[Vec3]) -> Result<Body> {
             splits: body.splits.clone(),
             face_sources: None,
             history: None,
+            display_of: None,
             regularization_protected: true,
         };
         Ok(crate::record::rebuilt_body(split, &[&body.solid], &rec))
@@ -1053,6 +1054,7 @@ pub(crate) fn split_any(body: &Body, faces: &[usize], tool: &SplitTool) -> Resul
             splits: body.splits.clone(),
             face_sources: None,
             history: None,
+            display_of: None,
             regularization_protected: true,
         };
         Ok((crate::record::rebuilt_body(split, &[&body.solid], &imp.rec), imp))