Hydra follow-ups: group totals in the executor, Pass 1 HydraCms candidates, Hydra top-k #703

Scope (deferred for now)

Remaining Hydra work, postponed so the migration's core groups land first:

1. Group totals in the executor. Upstream asap_sketchlib Design: pre-ASAP structural CSE via bottom-up hash-consing over Rc<QueryExpr> #223 adds Hydra::query_group_total (minimum across rows; never undercounts with non-negative counts; bound n_g + 4N/w at confidence 1−(1/4)^r level). The planner pin branch planner-pin/univmon-hydra puts it on top of the current executor pin. Executor wiring:
   • move the pin;
   • add HydraCms::group_total;
   • answer a point count with value: None in HydraCmsGroup::estimate;
   • accept value: None in capability.rs;
   • fix group_view_reads_and_merges_one_group.
2. Pass 1 Hydra (G7b). Offer HydraCms for grouped counts, with the Absolute ε·‖stream‖₁ guarantee (after the accuracy metric work in Accuracy targets and guarantees: three error metrics (absolute, relative, rank), a shared AccuracySpec, and metric-aware satisfies #690); prune the degenerate per-group Count-Min for COUNT(*) GROUP BY k with a reason. The executor side (HydraCms kernel, weighted updates, filtered aggregates) is already in feat(asap-physical-operators): execute HydraCms grouped counts and filtered aggregates #695.
3. Hydra top-k. A per-subpopulation heap kernel and its guarantee.

Related: #695, #690, #669, upstream sketchlib #223.