Fix direct outbound TUN loopback before inbound start

[leekery]

Summary

• seed the direct outbound loopback ranges from Karing's configured TUN prefixes before the TUN interface starts
• retain interface discovery as a fallback when no configured TUN prefix is available
• add regression coverage for an address inside the TUN prefix, an unrelated LAN address, and configurations without a TUN prefix

Problem

Karing starts outbounds before the TUN inbound. During direct outbound startup, InterfaceMonitor().MyInterfaces() can therefore still be empty, leaving myAddresses unset. A packet routed to direct whose destination is the TUN peer can then re-enter the TUN repeatedly. This was observed with UDP traffic to ports 5351 and 1900, but the fix intentionally protects the entire configured TUN range rather than hard-coding ports.

For a TUN address of 10.20.0.1/30, the regression test verifies that 10.20.0.2 is rejected before the physical interface exists while 192.168.1.254 remains allowed.

Related reports:

• [BUG] Thousands connections when turn on Naive Proxy with TUN karing#1642
• [Windows][TUN] direct outbound loops into the same TUN / direct 出站回环进入同一 TUN SagerNet/sing-box#4415

Testing

• go test -tags with_karing ./protocol/direct ./adapter/inbound -count=1
• go vet -tags with_karing ./protocol/direct