Adds embed(texts, model_override, fallback_order) to LLMRouter. Only
openai_compat backends are tried (Ollama/vLLM expose /v1/embeddings;
anthropic and vision_service do not). Uses embedding_model from backend
config when present, falls back to the chat model otherwise. Supports
cf-orch allocation and raises RuntimeError when all backends are exhausted.
4 tests added (TDD: RED → GREEN), 763 total passing, no regressions.
