perf(planner): collapse topk(sum by ...) into CMS-with-heap #744

Goal

Optimize the already-correct query-time implementation of:

topk(k, sum by (...) (metric))

into one CountMinSketchWithHeap precompute.

Intended plan

• Use CountMinSketchWithHeap.
• Set its aggregated labels to the inner sum by (...) labels.
• Use sum weighting/subtype.
• Emit no query-time aggregation pipeline for this optimized shape.

Prerequisite

The generic nested-aggregation path must exist first, with sum by (...) (metric) as the planned anchor and query-time topk. It is the correctness baseline for this optimization.

Validation

Benchmark the CMS-with-heap path against that baseline and retain Prometheus-equivalence coverage.