nx_sketch_hllmap.nx

sketch_hllmap.nx -- Map<key, HLL> for "distinct count per key" queries. E.g. distinct-IPs-per-URL, distinct-events-per-user, distinct-terms- per-document. Compose a hash map with the HLL primitive: each map entry holds a separate HLL sketch. CAPABILITY STOMP vs DataSketches: DS HllMap supports add + estimate but does NOT support merge of two HllMaps (their docs flag this as future work). We ship merge on day one because: - HLL merge is register-wise max (cheap) - hash-map merge is set-union of keys (linear) together they compose without state surgery. Hash table: linear-probing, power-of-2 capacity, open addressing. Entry = {key_hash: i64, hll: *Hll}. Empty slots: key_hash == 0 (we map true-hash 0 -> 1 to avoid collision with the empty sentinel). Per the lossless-language doctrine: per-key estimates ship the same Approximate<i64> envelope as base HLL (rel_stddev = 1.04/sqrt(m)).