Top-k k is a fixed constant (TOPK_K = 32); make it a per-query variable #184

Problem

Top-k's k is the constant TOPK_K = 32 throughout:

• sketch-bench grades every top-k row at precision@32;
• the study measures top-k curves at 32 only;
• rqe-optimizer prices every top-k RAQE at k = 32: heap m·32, 32 output entries, and the theory bound for the top 32.

A PromQL topk(k, …) carries its own k. A query with k > 32 is planned optimistically: lower precision and a bigger heap than priced. One with k < 32 is planned pessimistically.

Design

• sketch-bench: a topk_k= config param on the heap top-k variants (default 32). The heap defaults to k, and a heap below k is refused. The answer is the heaviest k, and the comparator grades precision@k at it.
• Study: top-k curves run at each k in --topk-ks (default 10, 32, 100). k ≠ 32 is keyed in the config string (topk_k=k), so k = 32 keeps its old keys.
• rqe-optimizer:
  • Raqe.topk_k: Option<u64>, default 32;
  • heap = m·k, and eligibility requires heap ≥ (L/x)·k;
  • accuracy reads the curve at k, else at the next larger measured k; past every measured k, only the theory bound at k;
  • output cost is k entries.
• ASAPQuery: passing the literal k into Raqe.topk_k goes into the existing ASAPQuery #804 after this lands.

The AutoSketch vs. ASAP evaluation (all top-k k = 32) is unaffected.

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