Add TCP transport node-level integration tests

Five tests covering the full TCP transport stack at the node level:
two-node handshake, three-node chain convergence with bloom filter
reachability, mixed UDP+TCP transport coexistence, MMP link-dead
detection after connection loss, and reconnection after link death
via connect-on-send.

src/node/tests/mod.rs

@@ -14,6 +14,7 @@ mod handshake;
 mod routing;
 mod session;
 mod spanning_tree;
+mod tcp;
 mod unit;
 
 pub(super) fn make_node() -> Node {