Add SVCB and HTTPS record types (RFC 9460)

[joyider]

Part of Feature breadth / record-type coverage in the Enterprise DNS Roadmap.

Add authoritative support for the SVCB and HTTPS resource records (RFC 9460). These power HTTP/3 (Alt-Svc), Encrypted Client Hello (ECH), and modern service binding, and are increasingly requested.

More involved than SSHFP/TLSA because of SvcParams parsing/serialization (mandatory, alpn, port, ipv4hint, ipv6hint, ech, ...) and the AliasMode vs ServiceMode distinction. miekg/dns provides dns.SVCB / dns.HTTPS with SvcParam types to build on.

Scope

• zone/rtypes/svcb.go + https.go (HTTPS shares SVCB wire format) registered via Register()
• SvcParam round-trip (presentation <-> wire) with tests, including AliasMode (priority 0)
• API create/list/delete through the generic record routes
• AXFR/IXFR carries the new types
• DNSSEC signing covers the new RRsets
• OpenAPI / docs

Do this after the simpler types (SSHFP/TLSA) are in place. Depends on #37.