Relocate FMP link wire codec into proto/fmp

Move the FMP mesh-layer wire format (common prefix, encrypted/msg1/msg2
headers, and the build_*/inner-header codec fns) out of node/wire.rs and
into proto/fmp/wire.rs, so the whole FMP wire surface lives with its
subsystem, matching the proto/fsp/wire.rs layout. The wire module becomes
pub(crate) mod wire; callers reach it via crate::proto::fmp::wire.

Behavior-neutral: pure relocation plus import-path rewrites across the
node/peer consumers; no logic change. Full lib suite green at baseline.

src/node/tests/bootstrap.rs

@@ -3,7 +3,7 @@
 use super::*;
 use crate::EstablishedTraversal;
 use crate::config::{TransportInstances, UdpConfig};
-use crate::node::wire::{PHASE_MSG1, PHASE_MSG2};
+use crate::proto::fmp::wire::{PHASE_MSG1, PHASE_MSG2};
 use crate::transport::udp::UdpTransport;
 use crate::utils::index::IndexAllocator;
 use std::collections::HashMap;