Extend store contract tests: all accumulator types, keyed entries, DeltaSet exclusion

- Add SerializableToSink import so clone-fidelity tests compile on concrete types
- Clone fidelity tests for all 11 accumulator types: SumAccumulator,
  MinMaxAccumulator, DatasketchesKLLAccumulator, IncreaseAccumulator,
  MultipleSumAccumulator, MultipleMinMaxAccumulator, SetAggregatorAccumulator,
  DeltaSetAggregatorAccumulator, CountMinSketchAccumulator,
  CountMinSketchWithHeapAccumulator, HydraKllSketchAccumulator
- Three keyed-entry tests: grouping by key, coexistence of keyed/unkeyed, multiple keys per window
- DeltaSetAggregator cleanup exclusion test
- Concurrency tests: concurrent inserts (8 threads) and concurrent reads

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: zzylol