feat(xref): reference model with lexical paths, pathtypes, and statuses

Path normalization with stable keys, relative/absolute/none pathtypes, wildcard matching, and reference entries with loaded, stale, missing, and unreferenced states.

src/io/mod.rs

@@ -16,6 +16,7 @@ pub mod recovery;
 pub mod step;
 pub mod stl;
 pub mod xref;
+pub mod xref_model;
 pub mod linetypes;
 pub mod patterns;
 pub mod update_check;