feat(nat): Tailscale-inspired STUN/ICE + port mapping + mid-call re-gathering (#28)

Phase 8: 5 new modules bringing NAT traversal close to Tailscale's approach.

- stun.rs: RFC 5389 STUN client — public server reflexive discovery,
  XOR-MAPPED-ADDRESS parsing, parallel probe with retry, STUN fallback
  in desktop try_reflect_own_addr()
- portmap.rs: NAT-PMP (RFC 6886) + PCP (RFC 6887) + UPnP IGD port
  mapping — gateway discovery, acquire/release/refresh lifecycle,
  new PeerCandidates.mapped candidate type in dial order
- ice_agent.rs: candidate lifecycle — gather(), re_gather(),
  apply_peer_update() with monotonic generation counter,
  CandidateUpdate signal message forwarded by relay
- netcheck.rs: comprehensive diagnostic — NAT type, IPv4/v6,
  port mapping availability, relay latencies, CLI --netcheck
- relay_map.rs: RTT-sorted relay map, preferred() selection,
  populate_from_ack() for RegisterPresenceAck.available_relays

Relay: CallRegistry stores + cross-wires caller/callee_mapped_addr
into CallSetup.peer_mapped_addr. Region config + available_relays
populated from federation peers in RegisterPresenceAck.

Desktop: place_call/answer_call call acquire_port_mapping() and
fill caller/callee_mapped_addr. STUN+relay combined NAT detection.

571 tests pass (66 new), 0 regressions, 0 warnings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/src/config.rs

@@ -87,6 +87,14 @@ pub struct RelayConfig {
     /// Unlike [[peers]], no url is needed — the peer connects to us.
     #[serde(default)]
     pub trusted: Vec<TrustedConfig>,
+    /// Phase 8: geographic region identifier (e.g., "us-east", "eu-west").
+    /// Sent to clients in `RegisterPresenceAck.relay_region` so they can
+    /// build a relay map for automatic selection.
+    pub region: Option<String>,
+    /// Phase 8: externally-advertised address for this relay. Used to
+    /// populate `available_relays` in `RegisterPresenceAck`. If not set,
+    /// `listen_addr` is used.
+    pub advertised_addr: Option<SocketAddr>,
     /// Debug tap: log packet headers for matching rooms ("*" = all rooms).
     /// Activated via --debug-tap <room> or debug_tap = "room" in TOML.
     pub debug_tap: Option<String>,
@@ -114,6 +122,8 @@ impl Default for RelayConfig {
             peers: Vec::new(),
             global_rooms: Vec::new(),
             trusted: Vec::new(),
+            region: None,
+            advertised_addr: None,
             debug_tap: None,
             event_log: None,
         }