iter4: NVIDIA Cosmos real API integration (RFC-0002)

- Refactored to use shared infra: HttpClientBuilder, RetryPolicy, TokenBucket, AsyncJobRunner
- predict/generate/reason/embed/transfer all make real HTTP calls with auth
- NVCF async job pattern for generation (submit -> poll -> collect)
- Graceful fallback to deterministic output when no API key or API failure
- Rate limiting at 10 req/sec, exponential backoff with jitter
- All 219 unit + 48 integration tests passing

Author: AbdelStark