nx_sketch_kll.nx

sketch_kll.nx -- compactor-hierarchy quantile sketch. MRL99-style (Manku-Rajagopalan-Lindsay 1999) "compactor cascade": maintain levels 0, 1, 2, ... each holding up to k items. Items at level h carry weight 2^h. When level h fills, sort it, flip a coin, promote either even-indexed or odd-indexed items to level h+1 (the other half discarded). Karnin-Lang-Liberty 2016 (FOCS) tightens this via geometric per-level capacity decay; we use uniform capacity here for simpler code at slightly looser bound. Rank-error declared conservatively as ~1/sqrt(k) at conf 0.95. Versus Reservoir (sketch_reservoir.nx): KLL/MRL has tighter rank-error at fixed memory because the weighted-promotion cascade preserves rank information that uniform reservoir sampling discards. At k=200, MRL eps ~ 0.07 vs Reservoir eps ~ 0.085 (similar but slightly tighter); the win compounds at higher k. LOSSLESS-LANGUAGE DISCIPLINE (doc 20): Same NX_ENV_RANK_ERROR envelope as Reservoir; substrate doesn't double-count. Multiple primitives sharing an envelope kind is fine -- the discriminator is about the SHAPE of the error, not uniqueness.