* MLX: reduce Qwen3.8 Flash Next memory use
For NVFP4 creation, retain source precision for sensitive QSA projections
and non-expert MTP weights. Keep routed experts and PLE at NVFP4, and
promote other eligible weights to MXFP8.
Keep NVFP4 PLE tables host-backed, transferring only selected packed rows
and their scales to MLX. Honor scalar and per-expert NVFP4 global scales
in MoE, and per-shard global scales in host-backed PLE.
Use compressed QSA block selections in fused indexed Metal attention for
long prefill batches, retaining the graph fallback. Co-locate component
loading and forward methods, with explicit quantization and cache ownership.
* mlx: skip materializing host-backed tensors
Let models exclude tensors already owned by host storage from MLX loading.
Avoid allocating and discarding the full Qwen PLE tables at startup while
preserving the resident fallback.
* address comments