refactor(llm): Split layers.py by layer type #142

src/pygpukit/llm/
└── layers.py  (1491 lines - all layers)

src/pygpukit/llm/layers/
├── __init__.py          (exports)
├── base.py              (BaseLayer)
├── attention.py         (Attention, GQA, MHA)
├── mlp.py               (MLP, GatedMLP, SwiGLU)
├── norm.py              (RMSNorm, LayerNorm wrappers)
├── embedding.py         (TokenEmbedding, PositionEmbedding)
└── block.py             (TransformerBlock)