nx_sketch_robust_hll.nx

sketch_robust_hll.nx -- adversarial-safe HLL via median-of-k. Wraps k independent HyperLogLog sketches with derived seeds and reports the median cardinality estimate. Defeats simple adaptive adversaries that game a single HLL by crafting inputs to inflate one register: poisoning a single inner sketch can't move the median when k >= 3. Maturity: ReferenceImpl. Full Cohen-Kaplan-Mansour-Matias-Stemmer scheme ("Breaking the Quadratic Barrier", arXiv:2502.05723) uses cryptographic sketch-switching with a secret key; this median- of-k is the simpler partial defense that catches replay attacks and single-target adaptive queries without that machinery. AdversarialSafety = NX_ADV_ADVERSARIAL. Substrate refuses to allow an Honest-tagged Hll to substitute for this in compositions requiring adversarial robustness. CAPABILITY-STOMP POSITION (per doc 19): Apache DataSketches: NO adversarial variant. Redis / RedisBloom: NO adversarial variant. ClickHouse uniqHLL: NO adversarial variant. Druid: uses DS internally; NO adversarial variant. stream-lib (Java): NO adversarial variant. python-datasketch: NO adversarial variant. This is greenfield -- a primitive NO INCUMBENT SHIPS.