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/plugin.ts

@@ -47,6 +47,7 @@ export interface BrainyPluginContext {
    * - 'embedBatch'         — Batch embedding engine (texts[] → vectors[])
    * - 'distance'           — Distance function overrides
    * - 'msgpack'            — Msgpack encode/decode
+   * - 'aggregation'        — AggregationIndex replacement (incremental aggregates)
    *
    * Storage adapter keys:
    * - 'storage:<name>'     — Custom storage adapter factory