docs(planner): cost by use, two latency versions, the mixed set, results

- §4: plans priced by use, w_cpu * AUC(CPU) + w_mem * AUC(memory), CPU
  elastic: ingest on ceil(rho) workers split by sample, compaction at
  window close, query jobs; memory as ingest, storage, compaction and
  query, each counted once; latency is a batch's longest chain (model in
  sketch-bench #188). A peak-billed cost model was dropped.
- §5: latency is reported, in two versions: no SLA, with each method's
  cost-latency frontier, and a batch latency SLA over {100 ms .. 10 s}.
- §3: PerQuery is the same MILP over each RQE's own candidates (no sharing).
- §6: the evaluated set is the mixed template set, with shared r in {1, 8}
  and metrics m in {1, 8, 16}; the dashboard set is not evaluated.
- §7: AutoSketch's planning time is search plus its measured benchmark
  (approxbench accuracy runs at 1e8 items); figures are the frontier, cost
  vs. SLA and planning time; absolute costs only.
- §8-§10: PRs (#188, ASAPQuery #812, #138 stacked on #188), decisions Q3,
  Q5-Q7, limitations (elastic CPU, compaction merge cost).
- §11: results on the synthetic mixed set.

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

Author: zzylol