[unit: providers] Research document

[jayfalls]

Summary

Full research document for the Providers unit covering 7 open architectural questions:

1. Routing strategies - Round-Robin, Sequential, Failover (with circuit-breaker). Deferred: weighted, latency-based, cost-optimized.
2. Streaming - Internal-only in this unit. Gateway uses streaming provider APIs but buffers full responses for NATS consumers.
3. Retry/Timeout - Exponential backoff with full jitter, base 1s, cap 8s, 2 retries. Per-provider configurable timeouts (60s/120s/300s defaults).
4. Token counting - Provider-reported only, normalised to inclusive model at adapter layer.
5. NATS payload schema - Versioned JSON Go structs (LLMRequest / LLMResponse) with normalised error codes.
6. Encryption - Envelope encryption per-provider DEK wrapped by master KEK for zero-downtime key rotation.
7. Model feature detection - Static capability catalog + dynamic model list fetch + manual override.

Delivered: design/units/providers/research.md