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/sshd/metadata.go

@@ -19,6 +19,11 @@ type metadata struct {
 	keepaliveInterval time.Duration
 	keepaliveTimeout  time.Duration
 	keepaliveRetries  int
+
+	tcpKeepalive         bool
+	tcpKeepaliveIdle     time.Duration
+	tcpKeepaliveInterval time.Duration
+	tcpKeepaliveCount    int
 }
 
 func (d *sshdDialer) parseMetadata(md mdata.Metadata) (err error) {
@@ -64,5 +69,11 @@ func (d *sshdDialer) parseMetadata(md mdata.Metadata) (err error) {
 		d.md.keepaliveTimeout = mdutil.GetDuration(md, "keepalive.timeout")
 		d.md.keepaliveRetries = mdutil.GetInt(md, "keepalive.retries")
 	}
+
+	d.md.tcpKeepalive = mdutil.GetBool(md, "tcp.keepalive")
+	d.md.tcpKeepaliveIdle = mdutil.GetDuration(md, "tcp.keepalive.idle")
+	d.md.tcpKeepaliveInterval = mdutil.GetDuration(md, "tcp.keepalive.interval")
+	d.md.tcpKeepaliveCount = mdutil.GetInt(md, "tcp.keepalive.count")
+
 	return
 }