feat: Archive mount as filesystem (FUSE)

[Snider]

Summary

Mount archives as a read-only filesystem for browsing.

Use Case

Browse large archives without extracting. Access individual files.

Commands

# Mount archive
borg mount archive.stim /mnt/archive

# Mount encrypted (prompts for password)
borg mount encrypted.stim /mnt/archive

# Unmount
borg unmount /mnt/archive

Implementation

Use FUSE (Filesystem in Userspace):

• Linux: libfuse
• macOS: macFUSE
• Windows: WinFsp

Features

• Read-only access
• On-demand decryption for STIM
• Lazy loading (don't extract until accessed)
• Seek support for large files

Acceptance Criteria

• FUSE mount implementation
• Read-only file access
• Directory browsing
• Transparent decryption
• Proper unmount handling

[google-labs-jules]

Jules is on it. When finished, you will see another comment and be able to review a PR.

[google-labs-jules]

Ready for a review! A PR has been created.