fix: rewrite seq/fec for federation-delivered packets

Federation media from different senders had conflicting seq numbers,
FEC block IDs, and Opus decoder state. The relay now assigns fresh
monotonic seq/fec_block/fec_symbol to all federation-delivered packets,
ensuring clients see a clean continuous stream regardless of sender changes.

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

crates/wzp-relay/src/federation.rs

@@ -140,6 +140,9 @@ pub struct FederationManager {
     peer_links: Arc<Mutex<HashMap<String, PeerLink>>>,
     /// Dedup filter for incoming federation datagrams.
     dedup: Mutex<Deduplicator>,
+    /// Per-room seq counter for federation media delivered to local clients.
+    /// Ensures clients see monotonically increasing seq regardless of federation sender.
+    local_delivery_seq: std::sync::atomic::AtomicU16,
     /// Per-room rate limiters for inbound federation media.
     rate_limiters: Mutex<HashMap<String, RateLimiter>>,
 }
@@ -164,6 +167,7 @@ impl FederationManager {
             metrics,
             peer_links: Arc::new(Mutex::new(HashMap::new())),
             dedup: Mutex::new(Deduplicator::new(DEDUP_WINDOW_SIZE)),
+            local_delivery_seq: std::sync::atomic::AtomicU16::new(0),
             rate_limiters: Mutex::new(HashMap::new()),
         }
     }
@@ -907,15 +911,26 @@ async fn handle_datagram(
         }
     }
 
-    // Deliver to all local participants
+    // Deliver to all local participants with rewritten seq/fec
+    // so the client sees a monotonic stream regardless of which federation sender
     let locals = {
         let mgr = fm.room_mgr.lock().await;
         mgr.local_senders(&room_name)
     };
+    if !locals.is_empty() {
+        let new_seq = fm.local_delivery_seq.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+        let mut local_pkt = pkt.clone();
+        local_pkt.header.seq = new_seq;
+        // Rewrite FEC block/symbol to match new seq so decoder doesn't see stale blocks
+        let frames_per_block = 5u16; // matches default FEC config
+        local_pkt.header.fec_block = (new_seq / frames_per_block) as u8;
+        local_pkt.header.fec_symbol = (new_seq % frames_per_block) as u8;
+        local_pkt.header.is_repair = false; // federation packets are source-only for local delivery
         for sender in &locals {
             match sender {
-            room::ParticipantSender::Quic(t) => { let _ = t.send_media(&pkt).await; }
+                room::ParticipantSender::Quic(t) => { let _ = t.send_media(&local_pkt).await; }
-            room::ParticipantSender::WebSocket(_) => { let _ = sender.send_raw(&pkt.payload).await; }
+                room::ParticipantSender::WebSocket(_) => { let _ = sender.send_raw(&local_pkt.payload).await; }
+            }
         }
     }