group: Fix interrupt_exist_connections for routed connections

[xxspa]

Fixes #4281.

Root cause

Connections routed to a selector (as a route target or route.final) enter through the ConnectionHandler path (Selector.NewConnection / NewPacketConnection). Neither branch of that path goes through Selector.DialContext / ListenPacket, so these connections were never registered in the selector's interrupt group. SelectOutbound then interrupted an empty set, and existing connections kept flowing over the previously selected outbound — making interrupt_exist_connections a near no-op for user traffic.

URLTest is not affected on this path because its NewConnection passes itself as the dialer, so dialing goes through URLTest.DialContext, which registers the connection.

Fix

• Register the inbound connection in the selector's interrupt group (as an external connection) at the top of NewConnection / NewPacketConnection, covering both the nested-handler branch and the connection-manager branch. Simply passing the selector as the dialer (URLTest-style) would not be enough: members that only implement ConnectionHandler (e.g. the dns outbound) need the handler branch, and connections handled by nested group members are only registered in the inner group.
• Add SingPacketConn / Group.NewSingPacketConn to common/interrupt so an N.PacketConn can be registered, mirroring the existing wrappers (with Upstream / ReaderReplaceable / WriterReplaceable pass-through).

Tests

New end-to-end tests in test/group_test.go (full box instance, SOCKS inbound, selector as route.final, direct members, local echo servers):

• TestSelectorInterruptRoutedConnection: an established TCP connection must be closed after SelectOutbound when interrupt_exist_connections is enabled — failed before this fix (read timed out), passes now.
• TestSelectorInterruptRoutedPacketConnection: a UDP session must be re-established over a new outbound socket after switching — failed before this fix (same source observed by the echo server), passes now.
• TestSelectorKeepRoutedConnection: with interrupt_exist_connections disabled, existing connections must survive switching — passes before and after, guarding the default semantics.

Note: the test module's go.mod / go.sum are currently behind the main module (missing entries for sing-tun/gtcpip/checksum imported by common/tlsspoof), so running the tests requires a go mod tidy in test/ first; that sync is intentionally not included in this PR.