Mocks `readBestGranularityRollups` with a controllable promise so the
peak in-flight count is observable at the helper boundary. Asserts
peak <= 4 across a 15-type fan-out and re-checks after each drained
resolver so a future regression that floods the pool on completion
would also surface. Mirrors the v1.4.40 W-POOL route-level pin in
`src/app/api/analytics/__tests__/route.test.ts`.