fix(camera): keep ortho depth precision constant across zoom

The orthographic near/far span was ±1000x the camera distance, so
zooming out ballooned the depth range and collapsed z-buffer precision:
coincident solids, meshes and wires then flipped draw order (meshes drew
in front of solids only when zoomed out, correct order when zoomed in).

Size the depth half-range from the drawing's extent instead — the bbox
diagonal on fit_to_bounds, the view height on a restored saved view —
and hold it constant as the camera zooms, so precision no longer depends
on distance. Falls back to the old distance-scaled range when unset.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/camera_ops.rs

@@ -230,6 +230,10 @@ impl Scene {
             projection: view::camera::Projection::Orthographic,
             yaw,
             pitch,
+            // Sized from the saved view height rather than a zoom-scaled range,
+            // so a restored view keeps stable depth precision. `view_height` is
+            // the visible extent; a few multiples cover the drawing's depth.
+            depth_half_range: (view_height as f32 * 4.0).max(1.0),
         })
     }