[ExecuTorch][WebGPU] Add Gemma 4 MTP operator and route support

[JCNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Add Gemma 4 MTP and speculative-decode source-closure tests #21678
• [ExecuTorch][WebGPU] Add Gemma 4 speculative decode runtime #21677
• [ExecuTorch][WebGPU] Add Gemma 4 MTP export path #21676
• -> [ExecuTorch][WebGPU] Add Gemma 4 MTP operator and route support #21675
• [ExecuTorch][WebGPU] Add plain Gemma 4 source-closure tests #21674
• [ExecuTorch][WebGPU] Add Gemma 4 plain export and artifact contract #21673
• [ExecuTorch][WebGPU] Add Gemma 4 plain runtime and guarded routes #21672
• [ExecuTorch][Vulkan] Support scoped Gemma symbolic partitioning #21671
• [ExecuTorch][WebGPU] Add shared model runtime prerequisites #21670

Add the backend-only operator surface required by Gemma 4 MTP k2_round. The diff introduces exact-shape staged TopK, duplicate-safe generic Scatter, a provenance-certified unique-index WG64 Scatter route, long-context 2D dispatch closure, integer subtraction metadata, and a guarded raw-fp32 M=3 Q4 path.

Key changes:

• TopK implements official fp32 [1,1,2048] to [1,1,32] values/indices with WG64 metadata.
• Scatter keeps aten.scatter.src duplicate-safe and exposes parallel execution only through certified et_vk.scatter_src_unique.default.
• Quantized linear/subtraction adds the MTP M=3 Q4 route and constant-scalar projection with generic fallback.
• Codegen strips comments before workgroup parsing and regenerates headers/registry entries.

MTP delegation is instance-scoped; plain Gemma exposes neither TopK nor certified Scatter.

Co-authored-with: Claude Code.

Differential Revision: D115234084

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❌ 2 New Failures, 1 Pending

As of commit 60a9937 with merge base ceca90f ():

NEW FAILURES - The following jobs have failed:

• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, models) / linux-job (gh)
  /pytorch/executorch/backends/webgpu/runtime/ops/to_copy/ToCopy.cpp:217:6: error: redefinition of 'add_bool_to_float_op'
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh)
  /pytorch/executorch/backends/webgpu/runtime/ops/to_copy/ToCopy.cpp:217:6: error: redefinition of 'add_bool_to_float_op'

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