SessionDatagram forwarding handler with coordinate cache warming

Add handle_session_datagram handler replacing the 0x40 dispatch stub.
Transit nodes now decode the datagram envelope, enforce hop limits,
warm coordinate caches from SessionSetup/SessionAck/DataPacket payloads,
route via find_next_hop, and generate CoordsRequired/PathBroken error
signals on routing failure.

14 new tests covering decode errors, hop limit enforcement, local
delivery, cache warming for all session message types, single-hop and
multi-hop forwarding through live node chains, error signal generation,
and cache warming enabling subsequent routing. 375 tests pass.

src/node/tests/mod.rs

@@ -6,6 +6,7 @@ use std::time::Duration;
 
 mod bloom;
 mod disconnect;
+mod forwarding;
 mod handshake;
 mod routing;
 mod spanning_tree;