Planner: error models and parameter lists for Hydra (and other unsized families), so Pass 1 can offer them #1228

Example

topk(10, sum by (job) (rate(http_requests_total[5m]))) evaluated for every one of 2,000 job values could share one Hydra-CMS structure (one sketch per subpopulation, shared rows and columns) instead of 2,000 separate Count-Min sketches. The executor can build and query Hydra structures, but the planner never offers one: it has no error model for Hydra, so it cannot say what (ε, δ) a given Hydra size guarantees for a subpopulation's estimate, and it cannot check the query's accuracy target.

Where this shows today

• Pass 1 offers only the families in FAMILY_PREFERENCE; estimators::size_params treats HydraCms, HydraCountSketch, HydraKll, HydraHll and HydraUnivMon as unreachable! (crates/accuracy/src/estimators/mod.rs).
• local_guarantee returns no guarantee for a Hydra state; a Pass 1 test asserts this.
• Under Tracking: implement the optimization formulation (memo candidates, properties, edges y_{u,c,i}, MILP) as named code per stage #1207 (PR 1, refactor(types): properties and requirements from #1207 (#1207 PR 1) #1227) every sketch build's θ must be in its family's predefined parameter_list. The Hydra lists are empty, so no Hydra build is a valid candidate. The same holds for HllHip, KllDynamic, SpaceSaving, CmsWithSpaceSaving, CountSketchWithSpaceSaving, CmsFold, CountSketchFold, CountSketchWithL2Norm, Elastic, Coco, Bloom, SetAggregator, UnivMonTwoTier and UnivMonQ.

What is needed

For each Hydra kind (and, separately, each other family above that should be offered):

1. Error model: the guarantee (metric, ε, δ) of one subpopulation's estimate as a function of the Hydra sizes (width, depth, shared_rows, shared_columns, and the per-subpopulation sketch's own θ), for each statistic it answers. Absolute ε for Hydra-CMS was the starting point discussed earlier.
2. Predefined list: parameter_list(family) entries for that family, and the per-family comparison satisfies uses (θ_u ⪰ floor).
3. Error propagation: the metric each readout reports, so the metric rule and constraint (K) apply (concepts doc, Section 4).
4. Pass 1 offering the family once 1–3 exist.

Scope

Known limitation of the #1207 refactor; future work, not part of #1207's merge unit.