Adds /__wrnexus/islands.js (the bootstrap) and the /__wrnexus/island/
prefix (mount runtime, island bundles, shared chunks) to all three
serving paths.
Dev reuses the browserArtifactPaths registry pattern from pipeline.ts.
Prod mirrors the clientModulesDir handler, including its filename
allowlist, so island names cannot escape the output directory.
The bootstrap is inert without a data-wrn-island marker, so island-free
pages still download nothing.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
