nx_recall_scale_gate.nx

nx_recall_scale_gate.nx -- SCALE-ENVELOPE gate for the recall stack (the F227 scale-law: fixture-proven-only tools with undeclared caps = false signals worse than gaps). Proves nx_ir_eval + nx_recall_fuse + nx_recall_rerank CORRECT at N=2048 (three orders of magnitude past the 5-doc fixtures) and DECLARES the envelope. exit 0 = pass, N = failed. DECLARED ENVELOPE (honest): metrics exact at any n (O(n) sums; i64 headroom); the fusion/ rerank ORDERING helpers are selection-sort O(n^2) -- correct at any n, sized for post-BM25 shortlists (<= ~4k candidates); a 100k+ SERP re-sort needs a heap/merge rung (future, named).