Add _get_gpu_memory_map() to configure optimal memory strategy:
- GPU: 95% of available VRAM (leaves 5% for CUDA overhead)
- CPU: Up to user-specified limit (--ram) or auto-detected
- Disk: Only as last resort when GPU+CPU are full

Update --ram help text to clarify it's the CPU offloading limit.

This provides better performance by prioritizing GPU, then CPU,
and only using slow disk offloading when absolutely necessary.