feat: add aggregation engine with incremental SUM/COUNT/AVG/MIN/MAX, GROUP BY, and time windows

Add a write-time incremental aggregation engine that maintains running
totals on every add/update/delete for O(1) read performance. Integrates
into brain.find({ aggregate }) for a unified query API.

Core features:
- AggregationIndex with defineAggregate()/removeAggregate() API
- Five aggregation operations: SUM, COUNT, AVG, MIN, MAX
- GROUP BY with multiple dimensions including time windows
- Time window bucketing: hour, day, week, month, quarter, year, custom
- Materialization of results as NounType.Measurement entities
- Debounced persistence of definitions and state to storage
- Definition change detection via FNV-1a hashing with auto-rebuild
- Infinite loop prevention for materialized entities
- 'aggregation' plugin provider key for native acceleration
- Lazy initialization (created on first defineAggregate() call)
- 73 tests (unit + integration) covering all functionality

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

src/index.ts

@@ -26,9 +26,21 @@ export type {
   AddParams,
   UpdateParams,
   RelateParams,
-  FindParams
+  FindParams,
+  AggregateDefinition,
+  AggregateMetricDef,
+  AggregateSource,
+  AggregateQueryParams,
+  AggregateResult,
+  AggregationOp,
+  TimeWindowGranularity,
+  GroupByDimension,
+  AggregationProvider
 } from './types/brainy.types.js'
 
+// Export Aggregation Engine
+export { AggregationIndex, AggregateMaterializer, bucketTimestamp, parseBucketRange } from './aggregation/index.js'
+
 // Export zero-configuration types and enums
 export {
   // Preset names