[crisprintsstuff]

Fixes the two Poly Edit display problems a tester reported.

1. Edges showed through the model

The edited piece's polygon edges were painted by egui on top of the 3D view, culled only by face direction. After an extrusion, front-facing faces behind the new part still showed their edges, as if they were in front.

Now the renderer draws them in its existing depth-tested wireframe pass:

• DrawItem gains wireframe: bool, which draws one piece's edges even with View > Wireframe off.
• The edges use the user's wireframe color at opacity 0.9 (ITEM_WIREFRAME_OPACITY).
• Poly Edit sets the flag on its piece (Editor::poly_wire_piece) and no longer paints those edges itself.

Hover and reach highlights, creases and picks are still painted on top, as interaction feedback.

With the smooth preview on, the cage floats off the smoothed surface, so depth-testing it against that surface would hide most of it. In that mode the cage is still drawn on top, as before.

2. N-gons showed their inner lines

A mesh built from polygons told the renderer which edges are quad diagonals, but not which lie inside n-gons. The wireframe therefore drew the triangulation of every n-gon.

• Mesh now has set_ngon_edges / ngon_edges / is_inner_edge.
• They are filled when a mesh is built from a cage (Cage::build_mesh, relink) and when binding a Catmull–Clark stack whose base has n-gons (Multires::bind).
• The wireframe leaves those edges out, like quad diagonals.

This also covers meshes from the retopology tool and the View > Wireframe overlay.

Verified

• cargo fmt --check, clippy -D warnings, cargo test --workspace, check_corpus_ip.py.
• New tests:
  • An L-shaped prism with hexagon caps records 6 inner edges. Every triangle edge is either a polygon side or an inner edge, never both. A relinked cage restores them.
  • poly_wire_piece is on while Poly Edit is open and off with the smooth preview or after Done.
• Headless screenshots (lavapipe) of that prism in Poly Edit, from an angle where one arm hides the other:
  • The hidden edges aren't drawn.
  • The hexagon tops show only their six sides.
  • With the smooth preview, the cage outline is drawn on top as before.

These screenshots came from a temporary local hack (opening Poly Edit at start), which is not in this PR.

Not verified / notes

• Not run on a real GPU, Windows or macOS.
• The per-frame cost is one extra pass over the draw list. Inner edges are looked up only when a mesh's index buffers are rebuilt, i.e. on topology changes. No timing run.
• Meshes rebuilt by other tools (remesh, import) don't know their polygons, as before: quads are found from the shape, and n-gons don't exist there.
• In smooth-preview mode the cage is still drawn over the surface.

---

Generated by Claude Code