interrupt: Add wrapper for sing-style packet connections

Add SingPacketConn and Group.NewSingPacketConn to allow registering
N.PacketConn connections in an interrupt group, mirroring the existing
net.Conn and net.PacketConn wrappers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: zxspirit

common/interrupt/conn.go

@@ -4,6 +4,7 @@ import (
 	"net"
 
 	"github.com/sagernet/sing/common/bufio"
+	N "github.com/sagernet/sing/common/network"
 	"github.com/sagernet/sing/common/x/list"
 )
 
@@ -52,6 +53,31 @@ type PacketConn struct {
 	element *list.Element[*groupConnItem]
 }
 
+type SingPacketConn struct {
+	N.PacketConn
+	group   *Group
+	element *list.Element[*groupConnItem]
+}
+
+func (c *SingPacketConn) Close() error {
+	c.group.access.Lock()
+	defer c.group.access.Unlock()
+	c.group.connections.Remove(c.element)
+	return c.PacketConn.Close()
+}
+
+func (c *SingPacketConn) ReaderReplaceable() bool {
+	return true
+}
+
+func (c *SingPacketConn) WriterReplaceable() bool {
+	return true
+}
+
+func (c *SingPacketConn) Upstream() any {
+	return c.PacketConn
+}
+
 /*func (c *PacketConn) MarkAsInternal() {
 	c.element.Value.internal = true
 }*/

common/interrupt/group.go

@@ -5,6 +5,7 @@ import (
 	"net"
 	"sync"
 
+	N "github.com/sagernet/sing/common/network"
 	"github.com/sagernet/sing/common/x/list"
 )
 
@@ -36,6 +37,13 @@ func (g *Group) NewPacketConn(conn net.PacketConn, isExternal bool) net.PacketCo
 	return &PacketConn{PacketConn: conn, group: g, element: item}
 }
 
+func (g *Group) NewSingPacketConn(conn N.PacketConn, isExternal bool) N.PacketConn {
+	g.access.Lock()
+	defer g.access.Unlock()
+	item := g.connections.PushBack(&groupConnItem{conn, isExternal})
+	return &SingPacketConn{PacketConn: conn, group: g, element: item}
+}
+
 func (g *Group) Interrupt(interruptExternalConnections bool) {
 	g.access.Lock()
 	defer g.access.Unlock()