feat: support mieru UDP outbound

[enfein]

Note: because UDP transport uses a single goroutine to handle all incoming packets to the same destination port, it is slow and can't survive stress test. We may fix that in the future.

[wwqgtxx]

maybe should fix those problem, we have encountered this panic twice in CI.

eg: https://github.com/MetaCubeX/mihomo/actions/runs/19690634360/job/56405414712#step:6:922

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x95ee0]

goroutine 867 [running]:
internal/sync.(*HashTrieMap[...]).LoadAndDelete(0xe87300, {0xa71800, 0x4000d70d7c})
	/opt/hostedtoolcache/go/1.24.10/arm64/src/internal/sync/hashtriemap.go:311 +0x50
internal/sync.(*HashTrieMap[...]).Delete(...)
	/opt/hostedtoolcache/go/1.24.10/arm64/src/internal/sync/hashtriemap.go:360
sync.(*Map).Delete(...)
	/opt/hostedtoolcache/go/1.24.10/arm64/src/sync/hashtriemap.go:78
github.com/enfein/mieru/v3/pkg/protocol.(*baseUnderlay).RemoveSession(0x40009c6410, 0x40014a54a0)
	/home/runner/go/pkg/mod/github.com/enfein/mieru/[email protected]/pkg/protocol/underlay_base.go:169 +0x70
github.com/enfein/mieru/v3/pkg/protocol.(*StreamUnderlay).onCloseSession(0x40009c6410, 0x400174a300)
	/home/runner/go/pkg/mod/github.com/enfein/mieru/[email protected]/pkg/protocol/underlay_stream.go:288 +0xa0
github.com/enfein/mieru/v3/pkg/protocol.(*StreamUnderlay).RunEventLoop(0x40009c6410, {0xe61a28, 0x1c407a0})
	/home/runner/go/pkg/mod/github.com/enfein/mieru/[email protected]/pkg/protocol/underlay_stream.go:203 +0x3bc
github.com/enfein/mieru/v3/pkg/protocol.(*Mux).newUnderlay.func1()
	/home/runner/go/pkg/mod/github.com/enfein/mieru/[email protected]/pkg/protocol/mux.go:648 +0x44
created by github.com/enfein/mieru/v3/pkg/protocol.(*Mux).newUnderlay in goroutine 275
	/home/runner/go/pkg/mod/github.com/enfein/mieru/[email protected]/pkg/protocol/mux.go:646 +0x584

[enfein]

The test error exposes some concurrency issue under stress. I will do some analysis.

[wwqgtxx]

Feedback indicates that the current UDP outbound traffic cannot handle cases where the server is a domain name.

I reviewed the existing client API and found that although a resolver can be specified, but it directly resolves the domain name to an IP address very early on via NewClientMuxFromProfile. This is unnecessary for TCP and cannot handle DDNS domain names for UDP (unless the client is restarted or reloaded).

Is there a better implementation, such as re-triggering the resolution when a reconnection occurs?

[enfein]

It is OK to pass domain name as is when creating Mux, and resolve the IP address inside NewStreamUnderlay and NewPacketUnderlay functions.