sketch_hllmap_merge_capability_bench.nx

sketch_hllmap_merge_capability_bench.nx -- capability-EXCEED bench. CLAIM TO VALIDATE: Apache DataSketches HllMap supports add() and estimate() but NOT merge() of two HllMaps. Substrate ships merge via per-key HLL register-max + key-union. This bench DEMONSTRATES the merge operation produces results equivalent to the ground-truth "stream both into one map" workflow that DS users must do manually. Capability EXCEED, not a perf claim. WORKLOAD: Map A: 3 keys with 100/50/20 distinct values each. Map B: 3 keys with 75/30/40 distinct values; one key overlaps A. Merged := nx_hllmap_merge(A, B) Truth := add A's flat stream + B's flat stream into a single fresh map Per-key estimates of Merged vs Truth must agree within HLL's 3-sigma band. MEASUREMENT: For each key in the merged keyspace, compare merged estimate vs truth estimate via nx_cmp_accuracy. All comparisons must produce EQUIVALENT or BEATS (NEVER LOSES = the merge is mis-implemented).