refactor(ext/node): consolidate `node:fs` (part 8) (#32640)
## Summary
- Inline `_fs/_fs_readv.ts`, `_fs/_fs_realpath.ts`, `_fs/_fs_stat.ts`,
and `_fs/_fs_statfs.ts` into the main `ext/node/polyfills/fs.ts`
polyfill
- Extract shared stat utilities (`CFISBIS`, `convertFileInfoToStats`,
`convertFileInfoToBigIntStats`, types) into `internal/fs/stat_utils.ts`
to avoid circular dependencies with `_fs_fstat.ts` and `_fs_lstat.ts`
- Update imports in `_fs_fstat.ts`, `_fs_lstat.ts`, `handle.ts`, and
`promises.ts`
Continuation of #32631.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
