refactor(core): Split array.py - separate GPUArray and memory management #146

src/pygpukit/core/
└── array.py  (741 lines)

src/pygpukit/core/
├── array.py             (GPUArray class only, ~400 lines)
├── memory.py            (memory allocation, transfer)
├── dtype.py             (dtype handling, conversions)
└── factory.py           (array creation: zeros, empty, from_numpy)