chore: drop legacy comments and the last unused dead_code allows

* io/xref.rs — replaced the 7-line "BlockRecord::is_loaded turned out to
  be unreliable" rationale with a one-liner. The reverted experiment
  lives in git history (e88a946, 0996b75).
* refedit::RefEditSession.insert_handle — set at construction but never
  read; the "kept for future REFCLOSE DISCARD" TODO can come back via
  git history when that feature lands.
* scene::mod — dropped the placeholder MeshModel re-export; nothing
  outside `mesh_model` consumes the bare name and MeshLodSet is the
  pipeline-facing type now.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/mod.rs

@@ -28,8 +28,6 @@ pub use camera::Projection;
 pub use hatch_model::HatchModel;
 pub use image_model::ImageModel;
 pub use mesh_model::MeshLodSet;
-#[allow(unused_imports)]
-pub use mesh_model::MeshModel;
 pub use object::{GripApply, GripDef};
 pub use pipeline::uniforms::Uniforms;
 pub use pipeline::viewcube::{hit_test, CubeRegion, VIEWCUBE_DRAW_PX, VIEWCUBE_PAD, VIEWCUBE_PX};