feat: extend TCP keepalive support to all TCP-based listeners and dialers

Add configurable TCP keepalive (keepalive, keepalive.idle,
keepalive.interval, keepalive.count) to all TCP-based transport types:
tls, mtls, mtcp, ws/wss, mws/mwss, http2, ssh, sshd listeners and
tls, mtls, mtcp, ws/wss, mws/mwss, ssh, sshd dialers.

For types that already use "keepalive" for an app-level protocol
(grpc listener/dialer, ws/mws dialers, ssh/sshd dialers), the TCP
keepalive parameters are exposed under the tcp.keepalive.* prefix to
avoid ambiguity.

Also extract shared helpers into internal/net/keepalive.go:
- WrapKeepaliveListener: applies KeepAliveConfig on every accepted TCPConn
- ApplyKeepalive: applies KeepAliveConfig to a single TCPConn

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dialer/mtls/dialer.go

@@ -12,6 +12,7 @@ import (
 	"github.com/go-gost/core/logger"
 	md "github.com/go-gost/core/metadata"
 	xctx "github.com/go-gost/x/ctx"
+	xnet "github.com/go-gost/x/internal/net"
 	"github.com/go-gost/x/internal/net/proxyproto"
 	"github.com/go-gost/x/internal/util/mux"
 	"github.com/go-gost/x/registry"
@@ -75,6 +76,15 @@ func (d *mtlsDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialO
 			return
 		}
 
+		if d.md.keepalive {
+			xnet.ApplyKeepalive(conn, net.KeepAliveConfig{
+				Enable:   true,
+				Idle:     d.md.keepaliveIdle,
+				Interval: d.md.keepaliveInterval,
+				Count:    d.md.keepaliveCount,
+			})
+		}
+
 		conn = proxyproto.WrapClientConn(
 			d.options.ProxyProtocol,
 			xctx.SrcAddrFromContext(ctx),

dialer/mtls/metadata.go

@@ -11,11 +11,21 @@ import (
 type metadata struct {
 	handshakeTimeout time.Duration
 	muxCfg           *mux.Config
+
+	keepalive         bool
+	keepaliveIdle     time.Duration
+	keepaliveInterval time.Duration
+	keepaliveCount    int
 }
 
 func (d *mtlsDialer) parseMetadata(md mdata.Metadata) (err error) {
 	d.md.handshakeTimeout = mdutil.GetDuration(md, "handshakeTimeout")
 
+	d.md.keepalive = mdutil.GetBool(md, "keepalive")
+	d.md.keepaliveIdle = mdutil.GetDuration(md, "keepalive.idle")
+	d.md.keepaliveInterval = mdutil.GetDuration(md, "keepalive.interval")
+	d.md.keepaliveCount = mdutil.GetInt(md, "keepalive.count")
+
 	d.md.muxCfg = &mux.Config{
 		Version:           mdutil.GetInt(md, "mux.version"),
 		KeepAliveInterval: mdutil.GetDuration(md, "mux.keepaliveInterval"),