Qwen 3.5 MoE: Add Metal build target

[manuelcandales]

Adds make qwen3_5_moe-metal which builds the runner linked against the
Metal backend instead of CUDA:

• CMakeLists.txt: conditional metal_backend vs aoti_cuda_backend linking
• CMakePresets.json: add qwen3-5-moe-metal preset (Darwin only)
• main.cpp: guard CUDA includes/calls behind EXECUTORCH_BUILD_CUDA,
  route T=1 prompts to decode method (prefill has min seq_len=2)
• Makefile: add qwen3_5_moe-metal target

Authored with Claude.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• -> 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]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18881

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❌ 1 New Failure, 2 Unrelated Failures

As of commit 598c58f with merge base 5707e2a ():

NEW FAILURE - The following job has failed:

• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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