cdrom_image_viso: Fix more non-Windows breakage

[ChrisDeadman]

Summary

The latest VISO commits broke that functionality on Linux.

Commit 198db451a changed VISO root directory stat population to read from plat_dir_t's current entry instead of calling stat() directly. macOS already exposed the base directory as the initial current entry (via fgetattrlist), but the POSIX backend pre-loaded the first child entry instead, causing plat_dir_is_dir() to return false for the root and aborting the VISO mount.

Fix: stat the base directory path before entering the readdir loop and expose it as a synthetic "." current entry. plat_dir_read() advances past it normally. plat_dir_rewind() and plat_dir_count_children() restore the same state. No change to Windows or macOS backends; no change to callers.

Checklist

• Closes N/A
• I have tested my changes locally and validated that the functionality works as intended
• I have discussed this with core contributors already
• This pull request doesn't require changes to the ROM set
• This pull request doesn't require changes to the asset set

[bhjdhgcehgrehge]

It's fixed.