[ExecuTorch][WebGPU] Route exact prefill shapes to the BK64 GEMM

[JCNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Lift FastVLM dispatch limits and harden graph construction #21602
• [ExecuTorch][WebGPU] Cover Conv1d routes and dynamic manifests #21601
• [ExecuTorch][WebGPU] Support general groups=1 Conv1d #21600
• [ExecuTorch][WebGPU] Test byte-packed BOOL tails #21599
• [ExecuTorch][WebGPU] Support byte-packed BOOL storage #21598
• [ExecuTorch][Vulkan] Preserve persistent buffer mutations #21597
• [ExecuTorch][WebGPU] Preserve Linear and Q4 embedding params across dynamic resize #21483
• [ExecuTorch][WebGPU] Generate logical and arithmetic binary shader variants #21451
• [ExecuTorch][WebGPU] Generate extrema and unary shader variants #21450
• [ExecuTorch][WebGPU] Generate typed to_copy shader variants #21449
• [ExecuTorch][WebGPU] Make WGSL generation rollback-safe #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

Llama prefill drives the ordinary quantized-linear projections at a small set
of fixed batch-row counts, and the generic Steel schedule leaves throughput on
the table for them. This adds a frozen BK64 Steel quantized-GEMM route for the
exact accepted Llama ordinary-projection shapes at live M128, M508, and M512,
selected only when the capability and dynamic-route guards all pass. M511 and
other prefill sizes stay on the generic Steel schedule and M1 stays on bicol
decode, so the route fails closed outside its accepted shapes. Mirrors Vulkan
xplat/executorch/backends/vulkan/runtime/graph/ops/glsl/q4gsw_linear_gemm__w_4x8.glsl
(the 4-bit grouped-symmetric-weight GEMM), specialized here for the BK64 tile.

Key changes:

• runtime/ops/quantized_linear/q4gsw_steel_bk64.wgsl (+ generated header): the
  BK64-tiled q4gsw prefill GEMM kernel.
• QuantizedLinear.cpp, WebGPUUtils.h: exact-shape predicate, capability guard,
  and dynamic re-entry into and out of the BK64 route.
  @exported-using-ghexport

Differential Revision: D113171739

Differential Revision: D113171739

[pytorch-bot]

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