COW fs op to create workspaces

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On workspace creation, on file systems that support it, it might might be possible to use a CopyOnWrite syscall to make cloning much faster.

Obvs workspace setup isn't the main bottleneck right now, but it does take aprox 1 second (actually, I should also look into why it's taking that long, wrong globs?) and could potentially be instant

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COW Directory Cloning Research

APFS (macOS)

• cp -c -R src dst clones entire directory tree with COW semantics
• Near-instant, zero extra disk usage until either copy is modified
• Per-block COW: only modified blocks are duplicated, not whole files
• Underlying syscall: clonefile(2) from <sys/clonefile.h> — works on directories too
• cp -c fails fast if unsupported; no silent fallback to full copy

Linux

• Btrfs — cp --reflink=always -R. First-class COW, similar to APFS clones.
• XFS — supported since Linux 4.9 with reflink enabled (mkfs.xfs -m reflink=1). Same cp --reflink interface.
• ext4 — no reflink/COW support, no plans to add it.

Other

• ZFS — COW snapshots at dataset level (zfs clone), but no file/dir-level reflink equivalent.
• ReFS (Windows) — has block cloning API.
• NTFS — no COW cloning.

Implementation notes

• On macOS: call clonefile(2) directly or shell out to cp -c -R
• On Linux: use ioctl(FICLONE) / copy_file_range or cp --reflink=always -R
• Rust: no stdlib wrapper for either; would need libc/nix FFI or subprocess