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-proto/src/lib.rs

@@ -25,8 +25,9 @@ pub mod traits;
 pub use codec_id::{CodecId, QualityProfile};
 pub use error::*;
 pub use packet::{
-    HangupReason, MediaHeader, MediaPacket, MiniFrameContext, MiniHeader, QualityReport,
-    RoomParticipant, SignalMessage, TrunkEntry, TrunkFrame, FRAME_TYPE_FULL, FRAME_TYPE_MINI,
+    CallAcceptMode, HangupReason, MediaHeader, MediaPacket, MiniFrameContext, MiniHeader,
+    QualityReport, RoomParticipant, SignalMessage, TrunkEntry, TrunkFrame, FRAME_TYPE_FULL,
+    FRAME_TYPE_MINI,
 };
 pub use bandwidth::{BandwidthEstimator, CongestionState};
 pub use quality::{AdaptiveQualityController, NetworkContext, Tier};