fix(lib): route on the full four-byte message code

Dispatch read one byte of the code, so a code with a non-zero high byte
was handled as its low byte. Message::get_message_code now reads the
whole little-endian uint32 and handlers register by u32. PeerInit and
PierceFirewall, the only frames with a one-byte code, get their own
accessor for the listener's first frame.

soulseek-rs-lib/examples/stress.rs

@@ -481,7 +481,7 @@ impl SeederState {
             let Ok(init) = read_framed(&mut p) else {
                 return;
             };
-            if init.get_message_code() != 1 {
+            if init.get_init_code() != 1 {
                 return;
             }
         }
@@ -735,7 +735,7 @@ fn accept_file_connection(
         let Ok(mut init) = read_framed(&mut stream) else {
             continue;
         };
-        if init.get_message_code() != 1 {
+        if init.get_init_code() != 1 {
             continue;
         }
         init.set_pointer(5);