Function save_to_disk 

pub fn save_to_disk(path: &Path)

What: Persist the current official index to path as JSON.

Inputs:

• path: File path to write JSON to

Output:

• Writes JSON to disk; errors are logged but not propagated to avoid interrupting the UI.

Details:

• Serializes under a read lock and ensures parent directory exists before writing.
• Creates parent directory if it doesn’t exist (Windows-compatible).
• Logs write failures for debugging but doesn’t crash background tasks.
• Warns if the index is empty when saving.