#6113: support ipv6

only ipv6 machine not run <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR introduces a new `gateway.bind` mode (`dualstack`) intended to improve IPv6 support by allowing the gateway to bind to an IPv6 wildcard address, and threads the new enum value through the CLI, onboarding flows, config schema, and related type definitions. Main concerns are around correctness of “dualstack” semantics: in `src/gateway/net.ts` the mode currently resolves to `::`, which can still be IPv6-only on common Linux configurations, and several caller surfaces (daemon status probing and onboarding/control-UI link generation) still assume IPv4 loopback/IPv4 tailnet, which can lead to false “gateway unreachable” reports or unusable URLs on IPv6-only hosts. <h3>Confidence Score: 3/5</h3> - This PR is likely safe to merge, but has functional gaps around IPv6-only vs true dual-stack behavior and a few UX surfaces that may misreport reachability. - Most changes are enum/threading updates and should be low risk, but the core behavior change (`dualstack` -> `::`) does not guarantee IPv4 reachability on common Linux setups and some related features (status probing, Control UI link generation) still assume IPv4, which can cause real user-facing failures in the targeted IPv6 scenarios. - src/gateway/net.ts, src/cli/daemon-cli/shared.ts, src/commands/onboard-helpers.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->