Add net/iplist for shared IP-policy source loading.
- Accept local TSV/CSV file paths or HTTP(S) URLs (including Google Sheets).
- Auto-detect TSV vs CSV format from file content.
- Send Accept: text/tab-separated-values header for content negotiation.
- Parse the first column and ignore comments (#) and optional "network" header.
- Expand nested list URLs up to MaxNestedSources (2) levels.
- Support Basic Auth from URL userinfo without sending credentials in the URL.
- Persist cached URL responses across restarts via httpcache.
- Refresh periodically and preserve the known-good cache on fetch or parse failure.
- Validate entries as IP address, CIDR range, or domain name (Validate()).
- Use golib/https.NewInternalClient() through httpcache.NewWith.
- Add README with usage examples and entry format documentation.
