nx_sketch_space_saving.nx

sketch_space_saving.nx -- SpaceSaving top-K heavy hitters. Metwally-Agrawal-El-Abbadi 2005. Bounded set of (key, count, error) triples of capacity K. On add: if key tracked, increment; else if room, insert with count=1, error=0; else evict the minimum-count entry and install (new_key, min_count + count, min_count). Guarantee: any item with true frequency > N/K is tracked (no false negatives for heavy hitters). The reported count over-estimates by at most `error`; true_count is in [count - error, count] for tracked items. COMPLEMENTS CMS: - CMS: per-query frequency for ANY key (overestimate-only). - SpaceSaving: enumerate the top-K most frequent keys directly. Together they cover the full frequency-counting axis. API takes i64 keys (caller hashes strings to i64 via murmur3 if they have string-keyed streams). LOSSLESS-LANGUAGE DISCIPLINE (doc 20): nx_ss_query returns ApproxI64 with envelope_kind = NX_ENV_ABS; param_a holds the per-key error bound (count - error <= true <= count); conf_ppb = 1e9 (deterministic; not probabilistic).