refactor(llm): Split model.py by model architecture #141

src/pygpukit/llm/
└── model.py  (1501 lines - all models)

src/pygpukit/llm/models/
├── __init__.py          (exports)
├── base.py              (BaseModel, common interfaces)
├── qwen.py              (QwenModel, Qwen2, Qwen3)
├── llama.py             (LlamaModel)
├── moe.py               (MoE model support)
└── registry.py          (model registry, auto-detection)