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 <[email protected]>

src/modules/home/modify/refedit.rs

@@ -23,9 +23,6 @@ use crate::command::{CadCommand, CmdResult};
 /// Active REFEDIT session.  Lives in `DocumentTab::refedit_session`.
 #[derive(Debug, Clone)]
 pub struct RefEditSession {
-    /// The INSERT entity being edited (kept for future REFCLOSE DISCARD).
-    #[allow(dead_code)]
-    pub insert_handle: Handle,
     /// Name of the block being edited.
     pub block_name: String,
     /// Handle of the block record (owns the block entities).