feat(e2e): add performance benchmark E2E tests

Add E2E tests for performance benchmarking:
- Query latency tests (p50, p95, p99) for simple, JOIN, aggregation
- Sequential and concurrent throughput tests
- SafeQueryExecutor overhead measurement
- Query validation overhead measurement
- Memory usage tests (streaming, repeated queries)
- Connection pool reuse efficiency
- Registry lookup and health check speed
- Performance summary report

Implements Issue #43 Phase 4.

Author: Sakeeb91