Merge FMP wire + PromotionResult relocations into next

Forward-merge the master-line FMP-establish relocations onto next: the FMP
link wire codec into proto/fmp/wire.rs and PromotionResult into
proto/fmp/core.rs. The wire move was hand-resolved against next's wire
format (msg3/Msg3Header/build_msg3, FMP_VERSION=1, no spin-bit flag), so
next's behavior is unchanged. Behavior-neutral; full lib suite green at
next baseline.

src/node/tests/session.rs

@@ -999,7 +999,7 @@ fn build_ipv6_packet(
 
 #[test]
 fn test_identity_cache_populated_on_promote() {
-    use crate::peer::PromotionResult;
+    use crate::proto::fmp::PromotionResult;
 
     let mut node = make_node();
     let transport_id = TransportId::new(1);