feat(stun): STUN server deduplication and concurrent multi-provider resolution

[dweekly]

Summary

Addresses Item 2 of ROADMAP.md ("STUN server list provides no actual redundancy"):

• Deduplicated STUN server list: Removed redundant stun1.l.google.com (which resolves to the exact same IP as stun.l.google.com, providing zero IP-level failover while multiplying DNS stall costs). The server list now contains distinct independent providers on distinct networks: Google (stun.l.google.com:19302) and Cloudflare (stun.cloudflare.com:3478).
• Concurrent multi-provider resolution: STUN fallback now resolves all endpoints concurrently across worker threads using DispatchQueue.concurrentPerform and thread-safe collection. If one provider's DNS query stalls (e.g. against an unresponsive or slow resolver), the other provider's resolution completes in parallel rather than paying consecutive 30-second serial timeouts.
• Pre-resolved query path: Added internal stunGetPublicIP overload operating on pre-resolved sockaddr_storage, preserving zero DNS delay between retries.

Stacked on top of #81 (feat/seed-public-ip).

Testing

• Unit tests in STUNTests.swift:
  • testSTUNServerList: Verifies multi-provider presence (Google + Cloudflare) and asserts no duplicate hostnames exist.
  • testResolveSTUNServer: Verifies resolution succeeds for valid targets and fails cleanly for invalid hosts.
  • All 40 STUN tests and 334 repository tests pass.
• Formatting checked with swift format lint --strict.
• DocC documentation builds warning-clean with --warnings-as-errors.