fix(select): pick 3D solids by clicking their body, not just edges

Single-click selection only tested wire-edge proximity, so clicking a
solid's face missed — only its thin projected edges were selectable.
Add mesh_click_hit: project each mesh triangle to screen and test the
cursor against it (front-most wins), wired as a fallback after the wire
and hatch hit-tests. Works for any meshed solid.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/update.rs

@@ -2850,6 +2850,11 @@ impl OpenCADStudio {
                                         vp_mat,
                                         bounds,
                                     )
+                                })
+                                .or_else(|| {
+                                    // 3D solids: click anywhere on the shaded
+                                    // body, not just the thin projected edges.
+                                    self.tabs[i].scene.mesh_click_hit(p, vp_mat, bounds)
                                 });
                             if let Some(handle) = hit {
                                 // Individual picks accumulate (issue #47):