feat(p2p): Phase 4 cross-relay direct calling over federation

Teaches the relay pair to route direct-call signaling across an
existing federation link. Alice on Relay A can now place a direct
call to Bob on Relay B if A and B are federation peers — the
wire protocol, call registry, and signal dispatch all learn to
track and route the cross-relay flow.

Phase 3.5's dual-path QUIC race then carries the media directly
peer-to-peer using the advertised reflex addrs, with zero
changes needed on the client side.

## Wire protocol (wzp-proto)

New `SignalMessage::FederatedSignalForward { inner, origin_relay_fp }`
envelope variant, appended at end of enum — JSON serde is
name-tagged so pre-Phase-4 relays just log "unknown variant" and
drop it. 2 new roundtrip tests (any-inner nesting + single
DirectCallOffer case).

## Call registry (wzp-relay)

`DirectCall.peer_relay_fp: Option<String>` — federation TLS fp
of the peer relay that forwarded the offer/answer for this call.
`None` on local calls, `Some` on cross-relay. Used by the answer
path to route the reply back through the same federation link
instead of trying (and failing) to deliver via local signal_hub.
New `set_peer_relay_fp` setter + 1 new unit test.

## FederationManager (wzp-relay)

Three new methods:
- `local_tls_fp()` — exposes the relay's own federation TLS fp
  so main.rs can build `origin_relay_fp` fields.
- `broadcast_signal(msg) -> usize` — fan out any signal message
  (in practice `FederatedSignalForward`) to every active peer
  link, returning the reach count. Used when Relay A doesn't
  know which peer has the target fingerprint.
- `send_signal_to_peer(fp, msg)` — targeted send for the reply
  path where the registry already knows which peer relay to
  hit.

Plus a new `cross_relay_signal_tx: Mutex<Option<Sender<...>>>`
field that `set_cross_relay_tx()` wires at startup so the
federation `handle_signal` can push unwrapped inner messages
into the main signal dispatcher.

## Federation handle_signal (wzp-relay)

New match arm for `FederatedSignalForward`:
- Loop prevention: drops forwards whose `origin_relay_fp` equals
  this relay's own fp (prevents A→B→A echo loops without needing
  TTL yet).
- Otherwise pulls the inner message out and pushes it through
  `cross_relay_signal_tx` so the main loop's dispatcher task
  handles it as if it had arrived locally.

## Main signal loop (wzp-relay)

### DirectCallOffer when target not local
Before falling through to Hangup, try the federation path:
- Wrap the offer in `FederatedSignalForward` with
  `origin_relay_fp = this relay's tls_fp`
- `fm.broadcast_signal(forward)` — returns peer count
- If any peers reached, stash the call in local registry with
  `caller_reflexive_addr` set, `peer_relay_fp` still None
  (broadcast — the answer-side will identify itself when it
  replies)
- Send `CallRinging` to caller immediately for UX feedback
- Only if no federation or no peers → legacy Hangup path

### DirectCallAnswer when peer is remote
- Registry lookup now reads both `peer_fingerprint` and
  `peer_relay_fp` in one acquisition
- If `peer_relay_fp.is_some()`:
  * Reject → forward a `Hangup` over federation via
    `send_signal_to_peer` instead of local signal_hub
  * Accept → wrap the raw answer in `FederatedSignalForward`,
    route to the specific origin peer, then emit the LOCAL
    CallSetup to our callee with `peer_direct_addr =
    caller_reflexive_addr` (caller is remote; this side only
    has the callee)
- If `peer_relay_fp.is_none()` → existing Phase 3 same-relay
  path with both CallSetups (caller + callee)

### Cross-relay signal dispatcher task
New long-running task reading `(inner, origin_relay_fp)` from
`cross_relay_rx`. In Phase 4 MVP handles:
- `DirectCallOffer` — if target is local, create the call in
  the registry with `peer_relay_fp = origin_relay_fp`, stash
  caller addr, deliver offer to local callee. If target isn't
  local, drop (no multi-hop in Phase 4 MVP).
- `DirectCallAnswer` — look up local caller by call_id, stash
  callee addr, forward raw answer to local caller via
  signal_hub, emit local CallSetup with `peer_direct_addr =
  callee_reflexive_addr` (peer is local now; this side only
  has the caller).
- `CallRinging` — best-effort forward to local caller for UX.
- `Hangup` — logged for now; Phase 4.1 will target by call_id.

## Integration tests

`crates/wzp-relay/tests/cross_relay_direct_call.rs` — 3 tests
that reproduce the main.rs cross-relay dispatcher logic inline
and assert the invariants without spinning up real binaries:

1. `cross_relay_offer_forwards_and_stashes_peer_relay_fp` —
   Relay A gets Alice's offer, broadcasts. Relay B's dispatcher
   creates the call with `peer_relay_fp = relay_a_tls_fp`.
2. `cross_relay_answer_crosswires_peer_direct_addrs` — full
   round trip; both CallSetups (one on each relay) carry the
   OTHER party's reflex addr.
3. `cross_relay_loop_prevention_drops_self_sourced_forward` —
   explicit loop-prevention check.

Full workspace test goes from 413 → 419 passing. Clippy clean
on touched files.

## Non-goals (deferred to Phase 4.1+)

- Relay-mediated media fallback across federation — if P2P
  direct fails (symmetric NAT on either side), the call errors
  out with "no media path". Making the existing federation
  media pipeline carry ephemeral call-<id> rooms is the Phase
  4.1 lift.
- Multi-hop federation (A → B → C). Phase 4 MVP supports a
  direct federation link between A and B only.
- Fingerprint → peer-relay routing gossip.

PRD: .taskmaster/docs/prd_phase4_cross_relay_p2p.txt
Tasks: 70-78 all completed

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

crates/wzp-proto/src/packet.rs

@@ -820,6 +820,38 @@ pub enum SignalMessage {
     ReflectResponse {
         observed_addr: String,
     },
+
+    // ── Phase 4: cross-relay direct-call signaling ────────────────────
+
+    /// Phase 4: relay-to-relay envelope for forwarding direct-call
+    /// signaling across a federation link. When Alice on Relay A
+    /// sends a `DirectCallOffer` for Bob whose fingerprint isn't
+    /// in A's local SignalHub, Relay A wraps the offer in this
+    /// envelope and broadcasts it over every active federation
+    /// peer link. Whichever peer has Bob registered unwraps the
+    /// inner message and delivers it locally.
+    ///
+    /// Never originated by clients — only relays create and
+    /// consume this variant.
+    ///
+    /// Loop prevention: the receiving relay drops any forward
+    /// where `origin_relay_fp` matches its own federation TLS
+    /// fingerprint. With broadcast-to-all-peers this prevents
+    /// A→B→A echo loops; proper TTL + dedup will land when
+    /// multi-hop federation is added (Phase 4.2).
+    FederatedSignalForward {
+        /// The signal message being forwarded
+        /// (`DirectCallOffer`, `DirectCallAnswer`, `CallRinging`,
+        /// `Hangup`, ...). Boxed because `SignalMessage` is
+        /// relatively large and JSON serde handles recursion
+        /// cleanly.
+        inner: Box<SignalMessage>,
+        /// Federation TLS fingerprint of the sending relay.
+        /// Used (a) for loop prevention by the receiver and (b)
+        /// to route the peer's reply back through the same
+        /// federation link via `send_signal_to_peer`.
+        origin_relay_fp: String,
+    },
 }
 
 /// How the callee responds to a direct call.
@@ -988,6 +1020,82 @@ mod tests {
         }
     }
 
+    #[test]
+    fn federated_signal_forward_roundtrip() {
+        // Wrap a DirectCallOffer inside FederatedSignalForward and
+        // prove both directions of serde preserve every field.
+        let inner = SignalMessage::DirectCallOffer {
+            caller_fingerprint: "alice".into(),
+            caller_alias: Some("Alice".into()),
+            target_fingerprint: "bob".into(),
+            call_id: "c1".into(),
+            identity_pub: [1u8; 32],
+            ephemeral_pub: [2u8; 32],
+            signature: vec![3u8; 64],
+            supported_profiles: vec![],
+            caller_reflexive_addr: Some("192.0.2.1:4433".into()),
+        };
+        let forward = SignalMessage::FederatedSignalForward {
+            inner: Box::new(inner),
+            origin_relay_fp: "relay-a-tls-fp".into(),
+        };
+        let json = serde_json::to_string(&forward).unwrap();
+        let decoded: SignalMessage = serde_json::from_str(&json).unwrap();
+        match decoded {
+            SignalMessage::FederatedSignalForward { inner, origin_relay_fp } => {
+                assert_eq!(origin_relay_fp, "relay-a-tls-fp");
+                match *inner {
+                    SignalMessage::DirectCallOffer {
+                        caller_fingerprint,
+                        target_fingerprint,
+                        caller_reflexive_addr,
+                        ..
+                    } => {
+                        assert_eq!(caller_fingerprint, "alice");
+                        assert_eq!(target_fingerprint, "bob");
+                        assert_eq!(caller_reflexive_addr.as_deref(), Some("192.0.2.1:4433"));
+                    }
+                    _ => panic!("inner was not DirectCallOffer after roundtrip"),
+                }
+            }
+            _ => panic!("outer was not FederatedSignalForward"),
+        }
+    }
+
+    #[test]
+    fn federated_signal_forward_can_nest_any_inner() {
+        // Sanity check that every direct-call signaling variant
+        // we intend to forward survives being boxed + re-serialized.
+        let cases: Vec<SignalMessage> = vec![
+            SignalMessage::DirectCallAnswer {
+                call_id: "c1".into(),
+                accept_mode: CallAcceptMode::AcceptTrusted,
+                identity_pub: None,
+                ephemeral_pub: None,
+                signature: None,
+                chosen_profile: None,
+                callee_reflexive_addr: Some("198.51.100.9:4433".into()),
+            },
+            SignalMessage::CallRinging { call_id: "c1".into() },
+            SignalMessage::Hangup { reason: HangupReason::Normal },
+        ];
+        for inner in cases {
+            let inner_disc = std::mem::discriminant(&inner);
+            let forward = SignalMessage::FederatedSignalForward {
+                inner: Box::new(inner),
+                origin_relay_fp: "r".into(),
+            };
+            let json = serde_json::to_string(&forward).unwrap();
+            let decoded: SignalMessage = serde_json::from_str(&json).unwrap();
+            match decoded {
+                SignalMessage::FederatedSignalForward { inner, .. } => {
+                    assert_eq!(std::mem::discriminant(&*inner), inner_disc);
+                }
+                _ => panic!("outer variant lost"),
+            }
+        }
+    }
+
     #[test]
     fn hole_punching_optional_fields_roundtrip() {
         // DirectCallOffer with Some(caller_reflexive_addr)