Poseidon wgpu/WASM speed-up backend

[jaehunkim]

Summary

This PR introduces an experimental WebBackend that off-loads the heaviest parts of Poseidon proving to the GPU via wgpu. In early tests we observe ≈ 2 × speed-ups on Apple M3 Pro (Chrome /WASM) and up to ≈ 2.3 × on Windows 11 (Ryzen 7 7800X3D + RTX 4070 Ti, Chrome /WASM) compared with the current SimdBackend.

more comments on hackmd : https://hackmd.io/@wiNfxgvfSc-Ln2DBfsvoqQ/SyY_THluxg

What’s in this PR

Area	Current Backend	New Backend (web)
Trace extension (extend_trace)	SIMD	GPU
Constraint evaluation on the evaluation domain (evaluate_constraint_quotients_on_domain)	SIMD	GPU
All remaining stages	Unchanged – stays on SimdBackend

• A new WebBackend type lives alongside the existing SimdBackend.
• The only compile-time branch between the two backends is in constraint_framework/src/component.rs – everything else is shared.
• For now, shaders must be supplied manually by downstream crates if they want to enable GPU off-load inside air.
• Referring to test_web_poseidon_prove() in crates/examples/src/poseidon/mod.rs is the easiest entry point for understanding how the WebBackend works.
  • It runs the same as the SimdBackend implementation until evaluate_constraint_quotients_on_domain, where it branches from SimdBackend.
  • It then calls FrameworkEval's eval() once to perform all evaluations in a single function call.

Benchmarks

• Windows 11 - Ryzen 7 7800X3D + RTX 4070 Ti

• Mac - Apple M3 Pro

• The first pair of graphs compares the total Poseidon-prove pipeline when run on the SimdBackend versus the new GPU-accelerated WebBackend.

• The next two graphs zoom in on the stages that are actually off-loaded to WebGPU—extend_trace and evaluate_constraint_quotients_on_domain—to show the raw shader speed-up in isolation.

How to Try It Out

cd crates/examples && wasm-pack test --chrome --release

• Using wgpu in a headless browser is currently unstable, so it wasn’t included in the CI.

[reviewable-StarkWare]

This change is 

[ohad-agadi]

this is very cool. I don't think this belongs in the main repo, as it will require support, which will give us a hard time.
perhaps it can be a separate crate that takes stwo as a cargo dependency. An 'extension' of sorts