feat(llm): add provider routing registry

[caozhengming]

Summary

Implements a stateless LLM provider routing layer for #99:

• adds Lux.LLM.Provider metadata for provider modules, models, capabilities, priorities, cost, latency, and quality signals
• adds Lux.LLM.ProviderRegistry for provider registration, capability filtering, strategy-based model selection, fallback chains, stable cache keys, and usage/cost summaries
• adds Lux.Prisms.LLM.ProviderRouterPrism so agents can request provider/model routing as a normal Prism
• exposes opt-in Lux.LLM.providers/0, Lux.LLM.select_provider/1, and Lux.LLM.call_provider/4 helpers without changing the default Lux.LLM.call/3 behavior
• documents provider routing in guides/llm_provider_routing.md

Verification

• git diff --check

I could not run mix test locally because this environment does not have Elixir/mix installed.