Add DNS-SD service discovery

[Abd002]

Description

Adds DNS-SD using cups/dnssd.h.

This is a CUPS 3-only API with no CUPS 2 equivalent, so the module is available only with #[cfg(cups3)].

API

Dnssd

• new(error_sender: Sender<String>) -> Result<Self>
• browse(...) -> Result<DnssdBrowser>
• resolve(...) -> Result<DnssdResolver>
• resolve_service(...) -> Result<DnssdServiceResolver>

Events and results

• DnssdBrowseEvent -> discovered service information.
• DnssdResolveEvent -> resolved service information.
• DnssdResolvedService -> resolved service information including its current addresses.

These types expose the relevant data reported by libcups, including service identity, host, port, TXT records, and addresses.

DnssdServiceResolver

try_recv(&mut self) -> Result<Option<DnssdResolvedService>>

Resolves a service's SRV/TXT records and its A/AAAA addresses, returning a combined result. A new result is emitted whenever the service's address set changes.

Example

examples/discover_system_services.rs demonstrates browsing and resolving _ipp-system._tcp and _ipps-system._tcp services.

When building against CUPS 2, the example provides a #[cfg(not(cups3))] stub so that cargo build --examples continues to succeed.