nx_citation_registry.nx

nx_citation_registry.nx -- the LIBRARIAN's external-reference rot guard. nx_librarian audits INTERNAL [[X]] doc links. This guards the EXTERNAL research references we ground the roadmap in (the rb_citation entries + module-header citations) as we build up from the hardware. The link-rot defence is structural: cite a STABLE scholarly anchor -- venue + year (POPL 2021), a DOI, an ISBN/book, a vendor doc number (Intel SDM), a datasheet, or an in-tree kernel doc path -- NOT a bare URL, which rots when the page moves. A reference with a permanent identifier is findable forever; a URL is not. The librarian ingests every roadmap reference and FAILS if any lacks a stable anchor. Cardinal #4 (verified facts) + the operator's standing rule: keep what we reference from rotting as the system grows bottom-up.