[SandObserver]

Summary

• browserLink drops a trailing dot from the host before its loopback checks, so localhost. and foo.localhost. give no link.
• It also treats :: and IPv4-mapped loopback or unspecified addresses (::ffff:127.x.x.x, ::ffff:0.x.x.x) as unreachable from the browser.

Testing
All green. Added 5 cases to the existing unreachable-address test.

Changelog

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.