Qwen 3.5 MoE: Add --backend metal export path

[manuelcandales]

Adds Metal backend support to export.py via --backend metal flag:

• _prepare_and_quantize_metal: applies source transforms, quantizes
  experts to MLX affine INT4, quantizes non-expert layers with fpa4w
  (skips shared_expert_gate with N<4 for prefill compatibility)
• _export_metal: exports decode + prefill methods via MetalBackend/
  MetalPartitioner

CUDA and MLX paths are unchanged.

Authored with Claude.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• Qwen 3.5 MoE Metal: Use max-sized prefill example for dynamic inputs #18956
• Qwen 3.5 MoE: Add Metal build target #18881
• -> Qwen 3.5 MoE: Add --backend metal export path #18880
• Qwen 3.5 MoE: Add Metal source transformations #18879
• Metal backend: Add gated delta rule kernel for linear attention #18878
• Metal backend: Add gather_qmv kernel for MoE expert-indexed quantized matmul #18877
• Metal backend: Add topk fallback kernel via MPSGraph #18876
• Metal backend: Add SDPA head_dim=256 support #18875
• Metal backend: Add INT32 and BOOL dtype support #18874

[pytorch-bot]

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