`__computeBunfsPackageRoot` now returns `//root/packages` for the Bun 1.3.14
`//root/<binary>` import.meta.dir shape, but production immediately joined that
root with shim and package segments through `path.join`, which collapses the
POSIX double-slash bunfs mount back to `/root`. That still made override
validation miss the embedded shim files.
Added a bunfs join helper that preserves the `//root` mount prefix after joining
production descendants, wired `bunfsPath` through it, and extended the #3329
regression test to assert the full typebox shim path stays under
`//root/packages/...`.
Fixes#3329
roboomp
03dae3814f