fix: prevent resolver from overwriting DNS cache hostname entries

[tito]

Summary

• When gost calls the resolver plugin with a raw IP address (common with tun2socks transparent proxying), net.LookupIP returns the IP itself. RegisterIPs then overwrites the correct hostname mapping (e.g. 160.79.104.10 -> api.anthropic.com) with a self-referential one (160.79.104.10 -> 160.79.104.10).
• This caused pending requests and request logs to progressively show raw IPs instead of hostnames, as each allowed connection corrupted its own cache entry.
• Fix: skip RegisterIPs when the host is already an IP address.

Root cause

The DNS cache is populated correctly by the DNS handler wrapper (dns_cache_handler.go) when a DNS query flows through the proxy. However, once the connection is allowed and gost dials the destination, gost calls the resolver plugin to "resolve" the target. For IP destinations, LookupIP("160.79.104.10") trivially returns [160.79.104.10], and RegisterIPs("160.79.104.10", ...) overwrites the correct entry.

Reproduction:

1. DNS query through proxy: api.anthropic.com -> 160.79.104.10 (cache correct)
2. First SOCKS5 connection to 160.79.104.10: cache HIT, shows api.anthropic.com (correct)
3. Resolver called during dial: overwrites cache with 160.79.104.10 -> 160.79.104.10
4. Second SOCKS5 connection to 160.79.104.10: cache HIT, shows 160.79.104.10 (wrong)

Test plan

• go test ./internal/greyproxy/... passes
• Manual test: DNS query + 3 sequential SOCKS5 connections all show correct hostname after fix (previously the 2nd+ connections showed raw IP)