Take the link frame overhead constant off a unix-only import

LINK_FRAME_OVERHEAD read ESTABLISHED_HEADER_SIZE through the
crate::node::wire import, which is #[cfg(unix)]. The constant feeds
link_wire_len, whose only caller send_session_datagram is compiled on
every platform, so the Windows build could not resolve the name and the
library failed with E0425.

Spell the path out in full instead of widening the import, which would
also pull FLAG_KEY_EPOCH, FLAG_SP and build_established_header onto
platforms that have no use for them. The module and the constant are both
unconditional, so only the use statement was ever the problem.

Found by GitHub CI, which builds Windows; the local gate is Linux-only
and cannot see this class of break at all. Of the eight symbols this file
imports only under cfg(unix), this was the one the security batch
referenced from ungated code.

src/node/handlers/session.rs

@@ -57,7 +57,13 @@ pub(in crate::node) const PATH_MTU_RELEASE_MIN_INTERVAL: std::time::Duration =
 /// wire: the established FMP header, the 4-byte session-relative timestamp and
 /// the AEAD tag. Mirrors the buffer `send_encrypted_link_message_with_ce`
 /// builds.
-const LINK_FRAME_OVERHEAD: usize = ESTABLISHED_HEADER_SIZE + 4 + crate::noise::TAG_SIZE;
+///
+/// Spelled out in full rather than through the `crate::node::wire` import
+/// above, which is `#[cfg(unix)]`. This constant feeds `link_wire_len`, whose
+/// caller `send_session_datagram` is compiled on every platform, so taking the
+/// name from that import fails to build on Windows.
+const LINK_FRAME_OVERHEAD: usize =
+    crate::node::wire::ESTABLISHED_HEADER_SIZE + 4 + crate::noise::TAG_SIZE;
 
 /// Wire size of an encoded `SessionDatagram` of `encoded_len` bytes.
 fn link_wire_len(encoded_len: usize) -> usize {