Fatal error during resolution through `RootLength`

@DanielRosenwasser がすでに取り組んでいます。

2026年6月8日 から。

• #4248 @copilot-swe-agent による — マージされずにクローズ

No repro, but error occurs at this line

```
panic(fmt.Sprintf("vfs: path %q is not absolute", p))
```

with this stack trace

```
internal/vfs/internal.RootLength()
internal/vfs/internal/internal.go:23
internal/vfs/internal.SplitPath()
internal/vfs/internal/internal.go:32
internal/vfs/internal.(*Common).RootAndPath()
internal/vfs/internal/internal.go:39
internal/vfs/internal.(*Common).Stat()
internal/vfs/internal/internal.go:47
internal/vfs/internal.(*Common).FileExists()
internal/vfs/internal/internal.go:59
internal/vfs/osvfs.(*osFS).FileExists()
internal/vfs/osvfs/os.go:104
internal/vfs/cachedvfs.(*FS).FileExists()
internal/vfs/cachedvfs/cachedvfs.go:71
internal/project.(*snapshotFSBuilder).FileExists()
internal/project/snapshotfs.go:311
internal/project.(*sourceFS).FileExists()
internal/project/snapshotfs.go:715
internal/module.(*resolutionState).tryFileLookup()
internal/module/resolver.go:1595
internal/module.(*resolutionState).tryFile()
internal/module/resolver.go:1580
internal/module.(*resolutionState).tryExtension()
internal/module/resolver.go:1568
internal/module.(*resolutionState).tryAddingExtensions()
internal/module/resolver.go:1530
internal/module.(*resolutionState).loadModuleFromFile()
internal/module/resolver.go:1425
internal/module.(*resolutionState).nodeLoadModuleByRelativeName()
internal/module/resolver.go:1391
internal/module.(*resolutionState).resolveNodeLikeWorker()
internal/module/resolver.go:575
internal/module.(*resolutionState).resolveNodeLike()
```