docs(planner): one p95 accuracy level; inputs from one sketch-bench study

- §5: a single p95 level in each family's metric (error ≤ 0.05, top-k
  precision ≥ 0.95) for synthetic and traces; the three strictness levels
  and fitted trace targets are dropped, and so is the grid's strictness
  dimension.
- §2/§6: costs and accuracy come from one study on asap_sketchlib 0.3.0:
  the cost table from --phase optimizer-cost (#174), a full-cross grid
  with the cost shape (#186), KLL merge curves (#179), top-k heap m·k
  (#182), per-query k with curves at 10/32/100 (#185) and the theoretical
  fallback (#180). Quantiles see Pareto a = 2, the cost shape.
- §6: traces are Alibaba 2022 and Google 2011; BOOM is left out for now.
- §8/§10: statuses as of 2026-10-07; the top-k m·k read is an assumption.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FhcWJExZcmVqS6r6rEtjis

Author: zzylol