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>

CLAUDE.md

@@ -24,6 +24,7 @@ Full architecture reference: `.claude/skills/architecture.md`
 - **Graph Engine** (`src/graph/`): Relationship traversal with adjacency index and pathfinding
 - **Metadata Index** (`src/utils/metadataIndex.ts`): O(1) exact match, O(log n) range queries
 - **Triple Intelligence** (`src/triple/`): Unified query combining all three intelligence types
+- **Aggregation Engine** (`src/aggregation/`): Write-time incremental SUM/COUNT/AVG/MIN/MAX with GROUP BY and time windows
 - **Virtual Filesystem** (`src/vfs/`): Full VFS with semantic search
 
 ### Type System