Pass 2 reuse rules: rollup, top-k reuse, nested sum merge #680

The legacy search could answer a computation from a cheaper related one. The planning stages don't do this yet. The results are correct; only cheaper plans are missing.

• Rollup: answer a coarse group by from a finer one (rollup).
• Top-k reuse: answer a small-k top-k from a larger-k one over the same partition (topk_reuse).
• Merge: collapse sum by (…) (sum_over_time(…)) into one accumulator.

These belong to Pass 2 (ASAP-aware CSE) as sharing rules, each producing a shared candidate that plan selection prices against the separate ones. The legacy implementations are at e80cfef (rollup.rs, topk_reuse.rs, and the nested-sum handling in replacement.rs) for reference.

Found in the #676 capability audit. Related: #580 and Pass 2.