node: register decrypt worker on cross-connection-won promotion

The cross-connection-won branch of `promote_connection` builds a
fresh ActivePeer with a new Noise session and our_index, inserts
it into peers, and registers identity, but did not hand the new
session to the decrypt shard worker pool. The normal-promotion
tail in the same function does make that call. A session
established via the cross-connection race path therefore missed
the worker fast-path for its lifetime, falling back to inline
decryption on the rx loop. Correctness was unaffected, but the
throughput/latency benefit of the worker pool was lost for peerspromoted through that path.

Mirror the normal-promotion tail and call
`register_decrypt_worker_session` after the fresh ActivePeer is
inserted into `self.peers` in the `this_wins` arm.

src/node/handlers/handshake.rs

@@ -1045,6 +1045,12 @@ impl Node {
                     "Cross-connection resolved: this connection won"
                 );
 
+                // Hand the FMP recv cipher + replay window to the
+                // decrypt shard worker. (Same as normal-promotion tail
+                // below.)
+                #[cfg(unix)]
+                self.register_decrypt_worker_session(&peer_node_addr);
+
                 Ok(PromotionResult::CrossConnectionWon {
                     loser_link_id,
                     node_addr: peer_node_addr,