[ExecuTorch][WebGPU] Add Gemma 4 MTP and speculative-decode source-closure tests

[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

Close Gemma 4 MTP and speculative-decode source contracts with production validation plus discriminating tests. This diff completes manifest and README closure alongside CPU authorities and native fixtures for TopK, Scatter, and M=3 Q4; model/oracle coverage for k2_round; runtime lifecycle contracts; and OSS, mirror, and CI guards.

Key changes:

• Production manifest validation binds source receipts, staged identities, and direct or legacy MTP views fail-closed.
• Operator and model suites cover malformed inputs, duplicate-safe versus certified Scatter, assistant and target positions, reset replay, and oracle reconciliation.
• Runtime tests require both factories, all wall/profile identities, teardown, and Emscripten closure.
• README and CI registration document and enforce the reproducible source workflow.

The diff does not promote pending manifests, partial MTP receipts, unsafe single-pass activation, or any benchmark result.

Co-authored-with: Claude Code.

Differential Revision: D115234079

[pytorch-bot]

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❌ 2 New Failures, 1 Cancelled Job

As of commit 4e3c9b4 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'

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-editable / linux / linux-job (gh)

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