Function cache_dir 

pub fn cache_dir() -> Result<PathBuf>

Returns the open-sesame cache directory, creating with secure permissions if needed.

Returns ~/.cache/open-sesame/ with 700 permissions. Fails when HOME is not set or directory cannot be created with proper permissions.

Security

• Never falls back to /tmp or other world-accessible locations
• Enforces 700 permissions on the directory
• Validates permissions on existing directories