nx_sketch_counting_bloom.nx

sketch_counting_bloom.nx -- Counting Bloom filter (Fan-Cao-Almeida-Broder 1998). Bloom filter where each "bit" is a c-bit COUNTER instead of a single bit. Insert increments k counters; delete decrements them. Query returns "present" iff all k counters > 0. Compared to other set-membership primitives we ship: - sketch_bloom (v2): bits only; no delete; smallest memory - sketch_cuckoo: stores fingerprints in buckets; supports delete; constant-time lookups; preferred when FPR < 3% - sketch_counting_bloom (this): COUNTERS not bits; supports delete; simpler than Cuckoo when FPR matters less; supports MULTI-SET membership (counter > N tests) FPR is the same as classic Bloom: (1 - e^(-kn/m))^k. Memory is k * c times Bloom's (c is counter width in bits). COUNTER SATURATION: caps at 2^c - 1. Once saturated, subsequent decrements DO NOT restore the original count (information loss). We use c=4 (saturation at 15), trading slight FPR overcount risk for memory. LOSSLESS-LANGUAGE DISCIPLINE: nx_cbloom_query returns ApproxI64 with NX_ENV_ABS, param_a tracking saturation events (so caller knows whether the filter has reached information-loss territory).