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>

docs/api/README.md

@@ -426,7 +426,8 @@ brain.defineAggregate({
     count:   { op: 'count' },
     average: { op: 'avg', field: 'amount' },
     highest: { op: 'max', field: 'amount' },
-    lowest:  { op: 'min', field: 'amount' }
+    lowest:  { op: 'min', field: 'amount' },
+    spread:  { op: 'stddev', field: 'amount' }  // Welford's online algorithm
   },
   materialize: true   // Optional: write results as NounType.Measurement entities
 })
@@ -441,7 +442,7 @@ brain.defineAggregate({
 | `source.where` | `Record<string, unknown>` | Metadata filter (same syntax as `find({ where })`) |
 | `source.service` | `string` | Multi-tenancy filter |
 | `groupBy` | `GroupByDimension[]` | Dimensions to group by — plain field names or `{ field, window }` for time bucketing |
-| `metrics` | `Record<string, AggregateMetricDef>` | Named metrics with `op` (`sum`, `count`, `avg`, `min`, `max`) and optional `field` |
+| `metrics` | `Record<string, AggregateMetricDef>` | Named metrics with `op` (`sum`, `count`, `avg`, `min`, `max`, `stddev`, `variance`) and optional `field` |
 | `materialize` | `boolean \| object` | Write results as `NounType.Measurement` entities (auto-visible in OData/Sheets/SSE) |
 
 **Time window granularities:** `'hour'`, `'day'`, `'week'`, `'month'`, `'quarter'`, `'year'`, or `{ seconds: number }` for custom intervals.