nx_dr_corpus_gate.nx

nx_dr_corpus_gate.nx -- KAT + neg-control for the file-backed corpus mode (DR-9). Reads REAL files off disk (this organ's own source and a sibling organ) rather than a fixture string, proving: a real document is read+tokenized, a phrase actually present in it is covered, a different file yields a different token count (so it is genuinely reading the file), per-file discrimination works, reads are deterministic, and the FAIL-LOUD neg-control -- a nonexistent path returns -1 instead of silently scoring zero coverage. Run from the nxc2 CWD (nx_sov_build_run). DRY nx_gate_verdict lib.