Relocate PromotionResult into proto/fmp

Move the PromotionResult enum (and its impl) out of peer::mod and into
proto/fmp/core.rs, alongside the cross_connection_winner tie-break helper
that was relocated the same way. This is FMP connection-lifecycle result
vocabulary, so it belongs in the FMP subsystem home rather than the peer
module.

Behavior-neutral pure type relocation: consumers import it from
crate::proto::fmp, and the crate-root public path crate::PromotionResult
is preserved via a re-export in lib.rs (mirroring cross_connection_winner).
Full lib suite green at baseline.

src/node/tests/session.rs

@@ -1002,7 +1002,7 @@ 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);