[Feature Request] Log the upstream proxy selection for multiple proxy configurations

**Description:** When multiple proxy upstreams are configured in the proxy list like this: ``` { "listen": [ "socks://127.0.0.1:1084", "socks://127.0.0.1:1086", "socks://127.0.0.1:1087", "socks://127.0.0.1:1088" ], "proxy": [ "https://username:password@domain.com", "https://username:password@ipv6.domain.com", "https://username:password@ipv6.additional.domain.com", "https://username:password@additional.domain.com" ], "log": "" } ``` the current log output only shows the destination address but lacks information about which upstream was selected for the connection. For users with complex setups (e.g., mixing IPv4/IPv6 upstreams), it's impossible to debug connection issues or monitor the health of specific upstreams via logs. **Proposed Solution:** Include the upstream proxy identifier (either the index or the domain/IP) in the connection log. **Use Case:** Monitoring performance and availability across different network paths (v4 vs v6) when using NaiveProxy as a core gateway on OpenWrt.