[ExecuTorch][WebGPU] Add HuggingFace rotate-half RoPE

[JCNTH]

Stack from ghstack (oldest at bottom):

• [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

Add HuggingFace rotate-half RoPE and migrate RoPE to shared dispatch construction

Qwen models pair the first and second halves of each head vector rather than adjacent elements. This adds the rotate-half operator with dynamic start-position updates, full-dimension Q/K handling, generated WGSL, and strict frequency-table bounds.

The shared RoPE handler also closes the dispatch-boilerplate review: it uses graph.device(), typed graph-owned parameter buffers, descriptor-driven bindings and pipelines, named validation and resize callbacks, generated shader-registry lookup, scoped ownership, and graph-owned workgroup recomputation. Interleaved shader math and output are unchanged.

Key changes:

• rotary_embedding_hf.wgsl and generated registry entry — one thread per pair for HuggingFace rotate-half.
• RotaryEmbedding.cpp — named validation, typed resize contexts, shared dispatch descriptors, and one initial/resize grid picker per route.
• Native tests — malformed input rejection, dynamic bounds, shrink/regrow reuse, and HF/interleaved lifecycle coverage.

Co-authored-with: Claude Code.
@exported-using-ghexport

Differential Revision: D113171746

Differential Revision: D113171746

[pytorch-bot]

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