[ExecuTorch][WebGPU] Repair dynamic SDPA routing, add attestation

[JCNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Generate logical binary shader variants #21451
• [ExecuTorch][WebGPU] Generate extrema shader variants #21450
• [ExecuTorch][WebGPU] Generate typed to_copy shader variants #21449
• [ExecuTorch][WebGPU] Make WGSL generation transactional #21448
• [ExecuTorch][WebGPU] Add Qwen3 K16 attention and fp16 KV cache #21136
• [ExecuTorch][WebGPU] Add HuggingFace rotate-half RoPE #21135
• -> [ExecuTorch][WebGPU] Repair dynamic SDPA routing, add attestation #21134
• [ExecuTorch][WebGPU] Select emdawnwebgpu for Emscripten builds #21133
• [ExecuTorch][WebGPU] Add Llama K16 online causal attention #21132
• [ExecuTorch][WebGPU] Replace broad QKV fusion with BK64 kernel #21131
• [ExecuTorch][WebGPU] Route exact prefill shapes to the BK64 GEMM #21130
• [ExecuTorch][WebGPU] Fuse the SwiGLU elementwise chain #21129
• [ExecuTorch][WebGPU] Coordinate dynamic dispatch routes #21128
• [ExecuTorch][WebGPU] Add certified scoped output suppression #21127

With dynamic sequence positions, the backend could record only one SDPA route,
so a decode following a dynamic-position prefill could dispatch the wrong
kernel, and there was no way to confirm which kernel actually ran. This treats a
dynamic SymInt position (not just a dynamic sequence dimension) as sufficient to
record both SDPA routes, exposes active-kernel route attestation for correctness
and performance harnesses, and reads the timestamp-query gate per execution so
diagnostics can be enabled after module initialization. The attestation state
and query are compiled only under the WGPU_BACKEND_ENABLE_PROFILING build flag,
so production builds carry no additional state or cost, mirroring how the Vulkan
backend gates its QueryPool behind ET_EVENT_TRACER_ENABLED. No Vulkan analogue
for the routing fix (WebGPU runtime routing and observability).

Key changes:

• WebGPUGraph.{cpp,h}: expose the active-route attestation query (compile-gated
  behind WGPU_BACKEND_ENABLE_PROFILING) and read the timestamp-query gate per
  execution.
• WebGPUUtils.h, Sdpa.cpp: treat a dynamic SymInt position (not only a dynamic
  sequence dimension) as sufficient to record both SDPA routes.
  @exported-using-ghexport

Differential Revision: D113171745

Differential Revision: D113171745

[pytorch-bot]

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