Ensure WASM artifact exists for static Pages/Preview deployments

[Copilot]

js/app.js imports wasm-fft/pkg/wasm_fft.js, but static publish flows can ship without pkg if WASM is not built first. This PR aligns local/preview deployment with the actual runtime requirement by making pkg generation explicit and repeatable.

• Preview publish flow: generate wasm-fft/pkg before copy/deploy

  • Adds Rust toolchain + wasm-pack setup in preview workflow.
  • Builds wasm-fft with --target web --release.
  • Fails fast with explicit diagnostics when pkg/wasm_fft.js is missing.

• WASM build reliability

  • Sets wasm-pack release profile to disable wasm-opt download dependency in wasm-fft/Cargo.toml for more predictable CI/local builds.

• Developer docs

  • Updates README usage flow to include wasm-fft build before serving the repo as static files.

# .github/workflows/PRview.yml
- name: Build wasm-fft package
  run: |
    cd wasm-fft
    wasm-pack build --target web --release
    if [ ! -f pkg/wasm_fft.js ]; then
      echo "ERROR: wasm-fft/pkg/wasm_fft.js not found after wasm-pack build"
      ls -la pkg || true
      exit 1
    fi