Rewrite api_types.rs as single source of truth with exact OpenAI field
names and object types:
- CompletionRequest/Response with logprobs, stop, penalties, seed
- ChatCompletionRequest/Response with tools, streaming chunks, deltas
- EmbeddingRequest/Response with encoding_format, dimensions
- ImageGenerationRequest/Response with size parsing, quality, style
- Model/ModelList with object fields ("model", "list")
- ErrorResponse envelope matching OpenAI error format
- ChatRole with as_str()/from_str() for wire format

Add router.rs — unified ModelRouter dispatching by model ID:
- complete() → GPT-2
- chat_complete() → OpenChat 3.5 (or GPT-2 via chat↔completion adapter)
- embed() → GPT-2 wte (extensible to Jina/BERT)
- list_models() / get_model() → all registered models
- chat↔completion adapters for cross-model compatibility

Rewire gpt2/api.rs, openchat/api.rs, stable_diffusion/api.rs to use
shared types. No JSON/serde surface — pure Rust types for in-binary
consumers (q2, lance-graph).

58 API tests passing.

https://claude.ai/code/session_01Y69Vnw751w75iVSBRws7o7