research: add Umami conformance test proving query-as-code advantage

Agent interview against umami-software/umami (23k+ stars). Ports all 10
query patterns (stats, pageviews, metrics, funnel, retention, journey,
goals, events, active visitors, attribution) to querymode DataFrame API.

Key finding: Umami's getAttribution runs 8 separate DB queries that each
rebuild the same CTE base. QueryMode does it in 1 collect() — 16ms at 10K
events. Five "impossible with Prisma SQL" tests demonstrate cross-report
correlation, anomaly detection, engagement scoring, and A/B testing.

38 tests, all passing. Benchmarks at 1K/10K/100K events.

Author: teamchong