feat(sketchdb): CMS-with-heap top-K accuracy bound - #57
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Splits `CountMinSketchWithHeap` out of the plain-CMS branch in `AccuracyProfile::derive` and gives it its own bound: * per-item frequency: e/w (CMS, Cormode-Muthukrishnan 2005) * top-K retention: 1/k (Metwally et al., ICDT 2005) → reported ε = max(e/w, 1/k); δ = 1/2^d (CMS depth) → new `AccuracyKind::TopK` variant (serialised as "top_k") Heap size `k` is read from `parameters.heap_size` (or aliases `topk` / `k`), default 100. ## Why split Plain CMS has *per-key point-lookup* accuracy; CMS-with-heap further guarantees *top-K retention* — items with true frequency ≥ N·ε are guaranteed to be in the returned heap. Conflating the two bounds under `AdditiveFrequency` lost the retention guarantee, which is what users actually query the heap for. ## Tests +4 on top of #55's 5: cms_with_heap_carries_top_k_kind_and_heap_bound cms_with_heap_cms_bound_dominates_when_heap_is_generous cms_with_heap_uses_default_heap_size_100 cms_with_heap_accepts_alternative_param_names 784 → 788 lib tests green; clippy + fmt clean.
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Splits
CountMinSketchWithHeapfrom plain CMS in the accuracy derivation; gives it a top-K retention bound from Metwally-Agrawal-El Abbadi (ICDT 2005) alongside the Cormode-Muthukrishnan CMS point-lookup bound. NewAccuracyKind::TopKvariant. ε = max(e/w, 1/heap_size), δ = 1/2^d. +4 tests, 788 green, clippy + fmt clean.