fix: omit inactive TUN IPv6 addresses

[SingLinkNetwork]

What changed

• Clear tun.inet6-address when the top-level configuration has ipv6: false.
• Preserve supplied TUN IPv6 addresses when IPv6 is enabled.
• Add real YAML round-trip regression tests for both paths.

Root cause

The client rewrites raw config before Mihomo runtime normalization. The old return path exposed an inactive default TUN IPv6 address alongside ipv6: false, producing a contradictory generated configuration.

Verification

• Static diff and Go formatting checks passed.
• The local linked worktree intentionally lacks the Clash.Meta submodule, so Go tests could not build locally and are not claimed as run.
• This PR must pass the full cross-platform CI suite before merge.

Closes #125