refactor: use wasm-runner

[sinui0]

Switch wasm test/bench runner to wasm-harness

Summary

Replaces the wasmtime --wasi threads runner with wasm-harness, which executes wasm32-wasip1[-threads] binaries inside a JS shell (V8 / SpiderMonkey) via a bundled WASI polyfill. Enables running cargo test / cargo bench against the same JS engines used in browser-targeted benches, without source changes.

Changes

• .cargo/config.toml
  • Added a [target.wasm32-wasip1] entry mirroring the threaded target's rustflags (atomics, bulk-memory, simd128, 4 GiB max memory).
  • Both wasm32-wasip1 and wasm32-wasip1-threads now use runner = "scripts/wasm-runner.sh".
• scripts/wasm-runner.sh (new, executable)
  • Auto-detects host core count (nproc on Linux, sysctl hw.ncpu on macOS, fallback 1) and exports WASM_HARNESS_ENV_RAYON_NUM_THREADS=<cores>.
  • wasm-harness rewrites WASM_HARNESS_ENV_X to X inside the guest, so rayon sees RAYON_NUM_THREADS without polluting the host shell.
  • Pre-existing WASM_HARNESS_ENV_RAYON_NUM_THREADS is respected (user override).
  • execs wasm-harness with cargo-supplied args.

Usage

Prerequisites: cargo install wasm-harness, then npm i -g jsvu && jsvu to install v8 and/or sm shells (defaults to whatever's first on $PATH / ~/.jsvu/bin).

cargo test  --target wasm32-wasip1
cargo bench --target wasm32-wasip1-threads
JS_SHELL=sm cargo bench --target wasm32-wasip1   # pick SpiderMonkey

Notes

• wasm-harness auto-forwards RAYON_*, CRITERION_*, RUST_*, NO_COLOR from the host into the guest; the WASM_HARNESS_ENV_ prefix is the explicit escape hatch used here.
• Bench code that builds its own rayon pool with an explicit num_threads() will ignore RAYON_NUM_THREADS — that's a per-bench concern, not a runner one.