feat(summary_executor): add coverage tracking and ExactAgg support

[zzylol]

Summary

Per the "one sid = one aggregation, read out directly" direction (no exact-vs-approximate special-casing, no outer-agg-fold stacking):

• Coverage: SummaryValue::{Points,TopK} now carry a trailing coverage: Option<(u64, u64)> — this group's own (min_window_end_ms, max_window_end_ms) observed, computed the same way ASAPTierResult.coverage actually is today (folded from raw window-END timestamps — confirmed the legacy code, despite its doc saying "min_window_start_ms", never actually tracks a window start; see sketch_reducer.rs's evaluate_core/evaluate_cardinality_global). Includes a carry-in base's window-end even though it never surfaces as an output point. Data_plane-local; nothing outside this file consumes it yet.
• ExactAgg: find_candidates/fetch_state/merge_states now also recognize AggKind::ExactAgg sids for SummaryKind::{Sum, Increase}. SidHandle/GroupState become enums with a parallel ExactAgg variant wrapping Arc<dyn AggregateCore> per window, reusing query_exact_agg_range's existing decode/merge machinery rather than reimplementing it. SummaryKind::MinMax/Count/Rate are deliberately not matched — MinMax's direction doesn't survive onto AggKind::ExactAgg's metadata, and no AggregationType resolves to Count/Rate today, so matching them would force a later caller to guess. readout()/SketchQuery still never see ExactAgg states (execute() stops at ExecOutcome::State first for these); the new GroupState::exact_value(&self, key) is the entry point a future live-serving caller uses to read that State's contents directly.

Both are pure additions — no change to what's wired into live serving (engine.rs still calls SketchReducer directly).

Test plan

• cargo build --workspace — clean
• cargo test -p data_plane --lib — 917 passed (0 failed); 21/21 in summary_executor including 6 new tests (2 coverage, 1 genuine carry-in-splice coverage test, 3 ExactAgg)
• cargo test -p data_plane --test e2e_controller_plans_and_backend_serves — 9/11 pass; the 2 known pre-existing failures reproduce identically on origin/main, unrelated to this change
• cargo clippy -p data_plane --lib — 75 warnings, identical count to baseline, none in the touched file
• rustfmt on the touched file

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