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/app/commands.rs

@@ -1890,7 +1890,6 @@ impl H7CAD {
                 }
 
                 let session = RefEditSession {
-                    insert_handle,
                     block_name: insert.block_name.clone(),
                     br_handle,
                     temp_handles: vec![],