feat(pick): select frame-like entities by clicking inside them

Images, OLE frames, wipeouts, underlays, paper-space viewports and
drawing-view borders were selectable only on their outline. Each now
carries an interior pick surface (pick_tris over its rectangle, or the
triangulated clip polygon for a clipped underlay), ranked below edge
and fill hits so content drawn inside still wins the click. The
layout's own sheet ("overall") viewport is excluded — identified by
the Layout object's viewport link, since DWG files carry viewport
id = 0 and some center the sheet viewport off-origin — or it would
swallow every click over the real viewports beneath it. Drawing-view
borders draw nothing normally; they emit an invisible pick-only wire
whose rect outline appears while selected.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/entities/underlay.rs

@@ -52,8 +52,12 @@ impl TruckConvertible for Underlay {
             }
             // Insertion grip.
             let key: Vec<[f64; 3]> = pts.clone();
+            // Interior pick surface over the clip polygon so the underlay
+            // selects on a click anywhere inside, not just on its outline.
+            let ring: Vec<[f64; 3]> = world_verts.iter().map(|v| [v.x, v.y, v.z]).collect();
+            let pick_tris = crate::entities::mesh::triangulate_planar(&ring);
             Some(TruckEntity {
-                pick_tris: Vec::new(),
+                pick_tris,
                 object: TruckObject::Lines(pts),
                 snap_pts: vec![(glam::DVec3::new(self.insertion_point.x, self.insertion_point.y, self.insertion_point.z), SnapHint::Node)],
                 tangent_geoms: vec![],