fix(config): avoid empty direct detours for DNS transports

[LiuLin1220]

Summary

• stop WARP-disabled DNS transports from detouring through a semantically empty direct outbound
• keep direct-fragment §hide§ available as a terminal route outbound
• add a real BuildConfig -> box.New -> box.Start -> Close regression test

Root cause

The generated dns-remote-no-warp, HTTPS dns-direct, and dns-trick-direct transports set DialerOptions.Detour to the fragment direct outbound. That outbound is valid as a terminal route action, but sing-box rejects it when it is initialized as an empty direct dialer detour:

detour to an empty direct outbound makes no sense

The first commit reproduces the failure across bare, explicit UDP, and HTTPS direct DNS addresses, with DNS fragmentation both disabled and enabled.

Change

Clear those DNS detours so the transports use their own direct dialer. The change preserves:

• the fragment direct outbound and tag used by terminal route actions
• trick DNS TLS fragment and record fragment settings
• the existing 300 ms trick DNS fallback delay
• sing-box's empty-direct validation

This PR does not change regional routing, user-rule behavior, Google routing policy, WARP endpoint topology, or sing-box validation.

Verification

Passed:

go test -count=20 ./v2/config -run 'TestBuildConfigStartsWithDirectDNSTransports|TestRawDNSDetourToEmptyDirectOutboundIsRejected'
go test -count=1 ./v2/config

Red/green evidence:

• test-only commit: all six startup cases fail with the expected empty-direct detour error
• fix commit: the same cases pass; the negative raw-config test still proves an explicit empty direct detour is rejected

Broader baseline comparison:

• go test -count=1 ./v2/... still reaches the pre-existing v2/profile/test.TestAddByContent failure because this build lacks with_wireguard; the same failure reproduces on db74dfc
• go vet ./v2/config still reports the pre-existing v2/config/warp.go:212:4: unreachable code; the same diagnostic reproduces on db74dfc

No workflow, Makefile, submodule, or release-build files are changed.