Function setup_cache 

pub fn setup_cache(
    cache_dir: &Path,
    verbosity: Verbosity,
    json_mode: bool,
) -> Result<()>

Ensure all required model assets are present in cache_dir.

For each asset:

1. If the file already exists (and has a reasonable size), skip it.
2. Otherwise, attempt to download it via curl (or wget as a fallback).

This function prints user-facing progress directly to stdout.