fix(net): defer DNS resolution to final dialer to preserve domain in proxy chain

Commit 0522ae8 added inline DNS resolution (PreferGo: true) in Resolve(),
resolving domain names to IPs before routing. This caused the SOCKS5
connector to send CONNECT with ATYP=IPv4 instead of ATYP=domain, breaking
chains where the backend (e.g., ssh -D) cannot reach the gost-resolved IP.

Revert the DNS fallback in Resolve() — return the address unchanged so
the original hostname flows through the proxy chain. Instead, add
net.Resolver{PreferGo: true} to the TCP dialer in internal/net/dialer,
which resolves DNS at the point of actual TCP connection — still pure
Go (no cgo thread exhaustion) but after the proxy chain routing is done.

Also rename resovle.go → resolve.go (fix typo).

Fixes go-gost/gost#877

internal/net/resolve.go

@@ -0,0 +1,63 @@
+package net
+
+import (
+	"context"
+	"fmt"
+	"net"
+
+	"github.com/go-gost/core/hosts"
+	"github.com/go-gost/core/logger"
+	"github.com/go-gost/core/resolver"
+	ctxvalue "github.com/go-gost/x/ctx"
+)
+
+// Resolve resolves addr to a concrete IP address. If hosts is non-nil, it is
+// consulted first. If r is non-nil and no host mapping matched, the resolver
+// is used. If neither is available, the address is returned unchanged so that
+// the original hostname is preserved for downstream connectors (e.g., SOCKS5
+// ATYP=domain). DNS resolution is deferred to the final TCP dialer, which uses
+// a pure-Go resolver to avoid cgo thread exhaustion.
+func Resolve(ctx context.Context, network, addr string, r resolver.Resolver, hosts hosts.HostMapper, log logger.Logger) (string, error) {
+	if addr == "" {
+		return addr, nil
+	}
+
+	host, port, _ := net.SplitHostPort(addr)
+	if host == "" {
+		return addr, nil
+	}
+
+	if log == nil {
+		log = logger.Default()
+	}
+	log = log.WithFields(map[string]any{
+		"sid": ctxvalue.SidFromContext(ctx),
+	})
+
+	if hosts != nil {
+		if ips, _ := hosts.Lookup(ctx, network, host); len(ips) > 0 {
+			log.Debugf("hit host mapper: %s -> %s", host, ips)
+			return net.JoinHostPort(ips[0].String(), port), nil
+		}
+	}
+
+	if r != nil {
+		ips, err := r.Resolve(ctx, network, host)
+		if err != nil {
+			if err == resolver.ErrInvalid {
+				return addr, nil
+			}
+			log.Error(err)
+		}
+		if len(ips) == 0 {
+			return "", fmt.Errorf("resolver: domain %s does not exist", host)
+		}
+		return net.JoinHostPort(ips[0].String(), port), nil
+	}
+
+	// Return the address unchanged so that the original hostname is
+	// preserved through the proxy chain. DNS resolution happens in the
+	// final TCP dialer (dialer.go) which uses net.Resolver{PreferGo: true}
+	// to avoid cgo-based lookups that block OS threads.
+	return addr, nil
+}