Build: revisit Turbopack support (currently pinned to --webpack)

`dev`/`build` are pinned to `--webpack` because Turbopack compiling the Miden SDK WASM graph exhausts memory (OOM'd a 48GB machine). Webpack also needs an `exportsPresence: false` shim for the v0.15 st/mt wasm-bindgen builds (see `next.config.ts`). Revisit when Turbopack handles the Miden WASM, or when Miden ships a Turbopack/Next equivalent of its Vite plugin. Tracking so we can drop the webpack pin + the export shim later.