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/spanning_tree.rs

@@ -126,7 +126,7 @@ async fn make_test_node_inner(config: Config, mtu: u16) -> TestNode {
 /// Sends msg1 over UDP. The drain loop will handle msg1 processing,
 /// msg2 response, and subsequent TreeAnnounce exchange.
 pub(super) async fn initiate_handshake(nodes: &mut [TestNode], i: usize, j: usize) {
-    use crate::node::wire::build_msg1;
+    use crate::proto::fmp::wire::build_msg1;
 
     // Extract responder info before mutably borrowing initiator
     let responder_addr = nodes[j].addr.clone();
@@ -288,7 +288,7 @@ pub(super) fn print_tree_snapshot(label: &str, nodes: &[TestNode]) {
 ///
 /// Returns the number of packets processed.
 pub(super) async fn process_available_packets(nodes: &mut [TestNode]) -> usize {
-    use crate::node::wire::{
+    use crate::proto::fmp::wire::{
         COMMON_PREFIX_SIZE, CommonPrefix, FMP_VERSION, PHASE_ESTABLISHED, PHASE_MSG1, PHASE_MSG2,
         PHASE_MSG3,
     };