Auto-build WASM packages on npm install via prepare hook

[secretrobotron]

After cloning the repo and running npm install, the npm run build command fails with this error:

[vite]: Rollup failed to resolve import "spark-rs" from "src/SplatPager.ts".
This is most likely unintended because it can break your application at runtime.
If you do want to externalize this module explicitly add it to build.rollupOptions.external.

(fresh clone on osx with cargo 1.94.1 and npm 11.8.0)

The error message suggests a Vite/Rollup config issue, but the real problem is that the spark-rs and spark-worker-rs WASM packages haven't been built yet. Their pkg/ directories are gitignored (correctly), but nothing in the standard setup flow builds them automatically. You have to know to run npm run build:wasm first, which is only mentioned in the README's build instructions in passing.

Fix

This PR sets the prepare script in package.json to npm run build:wasm. The prepare hook runs automatically after npm install, so the WASM packages are built before anything else needs them. This means npm install && npm run build just works out of the box for new contributors.

The WASM build is only triggered by npm install — not on every npm run build — so there's no performance cost during normal development iteration.

Verification

• Delete rust/spark-rs/pkg and rust/spark-worker-rs/pkg
• Run npm install — verify WASM packages are built automatically
• Run npm run build — verify it completes successfully

[dmarcos]

Thanks!

[mrxz]

While the change itself is okay, it's worth noting that this prepare script used to be there before. It was removed deliberately in commit 2fbde38. We should avoid changing this back and forth repeatedly.

AFAIUI there are two setups Spark is trying to accommodate: building/developing the project itself and using a specific commit of Spark as dependency in a project. The former requires the user to build the wasm projects. In this case having it part of the prepare script is beneficial. The latter is meant to use the pre-built bundles in the dist/ folder, but many package managers execute the prepare script regardless (see #195).

In practice people that wanted to test the latest commits ran into issues because they didn't have a Rust toolchain setup. Arguably this negates the convenience of providing pre-built bundles.

[secretrobotron]

@mrxz that totally makes sense. In that case, we should just revert, and update the README, no?