node: unregister old decrypt-worker entry on cross-connection-won promotion

The cross-connection-won path in handle_msg1 removes the old peer and frees
its allocated index, but does not unregister the old (transport_id, our_index)
cache_key from the decrypt worker pool. The orphan entry sits in the
per-shard HashMap until the index allocator recycles old_idx to a different
peer and that peer's register_decrypt_worker_session call overwrites it.
In the interim, any decrypt job that lands at the recycled cache_key
resolves to the wrong session and AEAD silently fails — observed as
multi-hop routing failure in 5-node static-mesh on next-branch where
bidirectional auto_connect drives cross-connections at every peer pair
on startup.

src/node/handlers/handshake.rs

@@ -1009,6 +1009,17 @@ impl Node {
                     (old_peer.transport_id(), old_peer.our_index())
                 {
                     self.peers_by_index.remove(&(old_tid, old_idx.as_u32()));
+                    // Unregister the OLD cache_key from the decrypt
+                    // worker pool BEFORE freeing the index for reuse.
+                    // Otherwise the worker's per-shard HashMap retains a
+                    // stale entry pointing at the removed peer's session;
+                    // if the index allocator later recycles old_idx to a
+                    // different peer, the new register call overwrites
+                    // the stale entry — but until that point, decrypt
+                    // jobs that land at the recycled cache_key resolve
+                    // to the wrong session and AEAD silently fails.
+                    #[cfg(unix)]
+                    self.unregister_decrypt_worker_session((old_tid, old_idx.as_u32()));
                     let _ = self.index_allocator.free(old_idx);
                 }