feat: add stddev/variance aggregation ops with Welford's online algorithm
- New aggregation ops: stddev and variance (incremental, O(1) per update)
- Welford's online algorithm for numerically stable running variance
- MetricState extended with m2 field for variance tracking
- AggregationProvider interface: defineAggregate, removeAggregate, restoreState, serializeState
- Export AggregateGroupState and MetricState types
- Plugin docs: aggregation provider, analytics providers (HyperLogLog, t-digest, etc.)
- Aggregation architecture and usage guide docs
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
David Snelling
b7c6752388