[ExecuTorch][WebGPU] Add shared model runtime prerequisites

[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

Provide the shared WebGPU runtime closure needed by Gemma plain/MTP and Qwen FlashDecoding. This adds build ownership, a transactional ordered-PTD model loader, fail-closed artifact validation, graph projection helpers, checked dispatch utilities, and long-row two-stage arg-reduce while keeping model adapters outside webgpu_backend.

Key changes:

• webgpu_model_loader validates ordered PTDs and required methods before publishing a loaded module.
• WebGPU graph/runtime utilities add checked buffer ranges, projection helpers, default-context ownership, and compile-gated timestamp-query support.
• Arg-reduce/codegen adds long-row partial/final reduction shaders and deterministic generated-header/registry checks.
• Buck/CMake defines backend, loader, utility, context, device, and profiling test targets.

Browser/dashboard code, model binaries, and model-specific orchestration remain outside this shared diff. Single-compute-pass capability is deliberately default-OFF; production activation requires a graph-safe capability certificate.

Co-authored-with: Claude Code.

Differential Revision: D115234032

[pytorch-bot]

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❌ 1 New Failure, 2 Unrelated Failures

As of commit ceafe14 with merge base ceca90f ():

NEW FAILURE - The following job has failed:

• pull / android / run-emulator (gh)
  adb: device offline

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, models) / linux-job (gh) (similar failure)
  /pytorch/executorch/backends/webgpu/runtime/ops/to_copy/ToCopy.cpp:207:6: error: redefinition of 'add_bool_to_float_op'

BROKEN TRUNK - The following job failed but was present on the merge base:

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

• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh) (trunk failure)
  /pytorch/executorch/backends/webgpu/runtime/ops/to_copy/ToCopy.cpp:207:6: error: redefinition of 'add_bool_to_float_op'

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