Copy public directory files verbatim instead of running the asset optimizer

[LucaCappelletti94]

Files in the public/ directory are documented as served as-is, but dx serve and dx bundle both ran them through the asset optimizer, and the two did not always agree. A .js entry gets inlined and minified, which shifts import.meta.url and can drop the entry's re-exports, so a worker module could work through dx bundle and fail under dx serve.

This bit us building a multi-threaded wasm app with wasm-bindgen-rayon (no-bundler feature). A build.rs emits the wasm-bindgen --target web glue plus a small loader.js entry into public/, and the entry re-exports the glue because the pool spawns blob workers that re-import it by URL and call pkg.default(...) and pkg.wbg_rayon_start_worker(...). Once those exports were optimized away, every pool worker threw TypeError: r.default is not a function and the SharedArrayBuffer pool never came up, while the same code ran fine through dx bundle. So it was a development-only break.

Copying public/ verbatim also stops optimizing public CSS and images, which matches the served-as-is contract (asset!() stays the path for assets you want optimized).