Conformance: add DNS-SD/mDNS integration tests on real network

[mark-e-deyoung]

Summary

Current tests validate mDNS contracts statically/unit-level. Add integration conformance checks against real multicast behavior.

Scope

• Add integration test harness (nightly or self-hosted runner) for RFC 6762/6763 behaviors
• Verify service registration, browse, TTL update, and graceful withdrawal
• Add collision/duplicate-name behavior checks
• Capture packet/log artifacts for failures

Acceptance Criteria

• Automated job verifies registration/discovery end-to-end
• Test artifacts include discovery logs and failure diagnostics
• Flaky test budget + retry policy documented

[mark-e-deyoung]

Release triage classification (2026-02-25): Defer unless blocked by a dependent project requiring real-network discovery conformance now.

Reason:

• Core discovery feature is implemented; this issue tracks deeper conformance validation.

Recommendation:

• Keep open, schedule after blocker set unless a consuming project requires it immediately.