We have a repo that uses bun for running scripts and somethings along with node in parallel since we aim to support both runtimes, we use pnpm as the package manager. So I have attempted to write something like this in package.json

  "devEngines": {
    "packageManager": { "name": "pnpm", "version": "10.20.0", "onFail": "download" },
    "runtime": [
      { "name": "bun", "version": "1.3.13", "onFail": "download" },
      { "name": "node", "version": "24.5.0", "onFail": "download" }
    ]
  }

this resolves pnpm as the package manager, and node version as runtime, but doesn't put the correct bun version on PATH. I expected this since Vite+ says it doesn't want to manage runtimes, but still allows to use bun as package manager. I asked in Voidzero discord and @TheAlexLichter confirmed too.
I still want to migrate our repo to Vite+, so I took a stab at this with help of Claude Fable 5, and the implementation came out at 350 lines, the mechanism for managing/downloading bun already exists, only the dev engines had to be hooked up. This doesn't mean Vite+ does everything through bun, it only makes it so the correct version of bun is available in path and the scripts in package.json and invoking bun from the shell downloads and uses correct bun version.

I have cleaned up the code and opened this PR on suggestion of @TheAlexLichter so we can have a discussion on this, the behaviour in this PR is what I exactly need. I can tweak how its implemented based on feedback.

Discord Thread https://discord.com/channels/1475973262193459293/1540305126269521941