fix: normalize IPv6 zones during IP parsing

[abczzz13]

Strip IPv6 zones before addresses reach trust and client-IP policy evaluation. Preserve zoned RemoteAddr compatibility, IPv4 unmapping, and parser round-trip stability.

Summary

Security/trust-boundary impact

Testing

• just test
• just race
• just lint
• just security
• just ci or explained why full local CI was not run

Checklist

• Added or updated tests where appropriate
• Updated README, godocs, or examples if needed
• Updated CHANGELOG.md if public behavior changed
• Considered backward compatibility
• Considered parser, proxy trust, and fallback security implications