Add Rspack plugins and Vue 2/3 infection (Rspack split 3/4)

What

Adds the Rspack plugins and the Vue 2/3 infection that complete a working Rspack bundle:

• Vue loader plugin: a custom matchResource replacement for vue-loader@15's crashing VueLoaderPlugin.
• Slim Rails manifest plugin: emits the manifest Rails consumes.
• Vue 2/3 @vue/compat infection: a loader plus a NormalModuleReplacementPlugin resolver, plus the shared scanner-graph edges they rely on.

Inert unless ENABLE_RSPACK is set.

Rspack / Webpack plugin-API interop

The main friction was porting the Vue 2/3 infection. Webpack propagates the ?vue3 marker through the module graph by mutating resolve data in an afterResolve hook. Rspack doesn't surface those resolve-mutation hooks the same way (it ignores beforeResolve mutations), so that propagation can't be reused directly. We reproduce it with two pieces: a loader that infects per-edge from the scanner's recorded import graph, and a NormalModuleReplacementPlugin resolver for the Vue-3-only node_modules packages the loader doesn't reach.

Verify Rspack works locally

This is the first part of the split that produces a complete, runnable Rspack bundle, so it can be verified end to end. Checked out on top of current master:

1. Install dependencies:

   yarn install

2. Build the production assets with Rspack:

   ENABLE_RSPACK=true NODE_ENV=production yarn build   # or simply: yarn rspack-prod

   Expected: Rspack compiles with 0 errors (a couple of warnings are expected), emitting minified, content-hashed, gzip-compressed bundles to public/assets/webpack/. This exercises the Vue loader, the slim manifest, and the Vue 2/3 infection end to end.

3. Serve that build through Rails and smoke-test in a browser:

   gdk config set webpack.enabled false && gdk reconfigure   # revert with `true`

   Open the key pages (MR diffs, repository, boards, dashboard) and confirm:

   • they render with no console errors;
   • Monaco (Web IDE / file editor) loads its workers via the native-ESM path;
   • any Vue 3 (@vue/compat) app populates rather than rendering empty (the infection check).

Only a manual production build is needed for this; the dev-server path (DEV_SERVER_BUNDLER=rspack), the Rails asset-pipeline integration, and CI coverage land in !243058 (merged).

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Part of splitting the opt-in Rspack build (!242376 (closed)) into reviewable pieces; !243058 (merged) (build

• dev-server wiring + CI) builds on this.

Epic: &22569 · Proposal: #604382 (closed)