chore: manifest cache improvements — eviction, immutability, async fs

Found during v0.5.21 release review. The manifest cache in `helpers.ts` has three design limitations:

1. **Single-entry cache** — silently evicts if user works across two dbt projects in one session
2. **Mutable shared objects** — callers could corrupt the cache (currently safe, but fragile)
3. **Synchronous fs calls** — blocks event loop on 100MB+ manifests

Deferred because: architectural change across dbt subsystem (>1 hour).