Design tab: studio lighting and body edges in the 3D view

Reported: once extruded, a part is hard to read; the lighting says
little about its shape. Both lights of the object shaders sit near the
camera, so the sides of a part come out in almost the same tone, and
nothing marks where one face ends and the next begins.

- The phong shader gains a studio lighting model, chosen by a new
  lighting_model uniform: a sky/ground hemisphere in world space (up
  faces cool and bright, down faces warm and dark), a key light from the
  upper left and a weak fill from the right, a plastic-like highlight,
  and a darker base with a faint sheen toward the silhouette so curved
  faces read as round. GLCanvas3D::set_studio_lighting() makes a canvas
  draw its objects with it whatever the realistic-view preferences; only
  the Design canvas turns it on. Every canvas sets the uniform on each
  use, 0 for the slicer's, so they render as before.
- Every B-rep edge of a body is drawn as a thin dark line over it, depth
  tested and pulled a few pixels toward the eye so it wins against the
  faces meeting at it and hides behind the faces in front. The seam of a
  closed surface and degenerate edges are left out
  (GeometryEngine::display_edges). Edges are sampled once per shape and
  kept across recomputes that leave a body unchanged; bodies faded by
  body focus get fainter edges, and a dress-up previewing its result
  alone hides them with the bodies.

Tests: display_edges gives a box its 12 edges at their lengths, a
cylinder its two round rims without the seam, a cone its base rim
without the seam or the apex.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QK4VgguuCAk2hZLWgcjJb9

docs/CAD/design_tab.md

@@ -362,6 +362,13 @@ the plane, `F` flips which half is kept. **Place on Face** (`F`, when section is
 picked face flat on the bed. Origin planes (`P`) and world axes (`A`) can be toggled on while
 you orient yourself.
 
+Bodies are lit as in a studio, whatever the realistic-view preferences of the slicer: faces
+facing up read cooler and brighter than faces facing down, a key light from the upper left
+separates the sides of a part, and curved faces darken toward their outline. Every edge of a
+body is drawn as a thin dark line, except the seam OCCT puts down the side of a cylinder or
+cone; the edges of bodies faded by body focus are fainter, and a dress-up previewing its
+result alone hides them with the bodies.
+
 ---
 
 ## Known limitations