Function ensure_cache_dir 

pub fn ensure_cache_dir(path: &Path) -> Result<()>

Creates the cache directory if it doesn’t exist and excludes it from backups.

This function:

1. Creates the directory and all parent directories if they don’t exist
2. Creates a CACHEDIR.TAG file to exclude from backup tools (borg, restic, etc.)
3. On macOS, marks the directory as excluded from Time Machine

This is idempotent - calling it multiple times on the same directory is safe.