feat: direct 1:1 calling via relay signaling (Phase 1)

New feature: call someone directly by fingerprint through the relay.

- Client connects with SNI "_signal" for persistent signaling
- RegisterPresence/RegisterPresenceAck for relay registration
- DirectCallOffer routed to target by fingerprint
- DirectCallAnswer with AcceptGeneric/AcceptTrusted/Reject modes
- Relay creates private room (call-{id}), sends CallSetup to both
- Both clients connect to private room for media (existing SFU path)
- Hangup forwarding + cleanup on disconnect
- Desktop CLI: --signal + --call <fingerprint> for testing
- CallRegistry tracks call state (Pending/Ringing/Active/Ended)
- SignalHub manages persistent signaling connections

Tested: Alice calls Bob by fingerprint, relay routes offer, Bob
auto-accepts, both join private room, media flows bidirectionally.

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

crates/wzp-relay/src/lib.rs

@@ -8,9 +8,11 @@
 //! quality transitions.
 
 pub mod auth;
+pub mod call_registry;
 pub mod config;
 pub mod event_log;
 pub mod federation;
+pub mod signal_hub;
 pub mod handshake;
 pub mod metrics;
 pub mod pipeline;