nx_doc_inref.nx

nx_doc_inref.nx -- incoming-reference counter for substrate docs. For each doc, counts how many OTHER docs contain a [[doc-name]] reference to it. Surfaces top-N most-referenced docs as data- driven V2 backfill priority targets. Composes nx_grep + nx_string_ops + nx_dir + nx_doc_census walker pattern. Substrate's first OPERATIONAL doc-graph analysis primitive -- moves beyond aggregate compliance to substrate-aware "which docs MATTER" visibility. **Quadrant:** REFERENCE (Diátaxis) **Topic Type:** REFERENCE (DITA) **Status:** DRAFT **Trust State:** WRITTEN_UNTESTED **Competitive State:** UNCLASSIFIED