[cdeil]

This code and PR description is from a coding agent.

Summary

• Move npm: externalization from Vite's deprecated alias customResolver into the existing Observable plugin.
• Run the resolver as a pre hook, preserving Notebook Kit's build and preview composition where plugins is assigned after spreading config().
• Add a focused test for plugin ordering, npm: resolution, externalization, and non-npm: fallthrough.

Why

Vite 8 warns that alias customResolver is deprecated and will be removed in Vite 9. The recommended replacement is a plugin with a resolveId hook and enforce: "pre".

Keeping the hook on observable() is important: Notebook Kit's build and preview commands replace the plugins property after spreading config(), so a separate plugin returned only by config() would be dropped.

Validation

Using Node 24 and pnpm 10, matching CI:

• Red/green focused regression test
• Full test suite: 25 files, 222 tests passed
• pnpm lint (TypeScript and ESLint)
• pnpm prepublishOnly
• Complete pnpm docs:build; real npm: imports resolve and the deprecation warning is absent
• Packed-package consumer build with Vite 8.2.2; the build passes without the warning