pqkem: clock data-path PSK rotation from WireGuard handshakes

Source OnDataPathRekeyed from the WGWatcher's per-handshake callback
(onWGCheckSuccess), which fires only on a fresh handshake, and OnDataPathDown
from the handshake-timeout path. A fresh handshake clocks the next chained
KEM exchange pushed over the data-path UDP transport.

client/internal/peer/conn.go

@@ -93,6 +93,11 @@ type PQHandshaker interface {
 	// SetRemotePort registers the peer's data-path endpoint from signalling: the peer's
 	// WG overlay IP combined with its advertised pq UDP port.
 	SetRemotePort(remoteKey string, overlayIP netip.Addr, port int)
+	// OnDataPathRekeyed signals a fresh WireGuard handshake for the peer; it clocks the
+	// next chained PSK rotation pushed over the data path.
+	OnDataPathRekeyed(remoteKey string)
+	// OnDataPathDown signals the peer's tunnel went down.
+	OnDataPathDown(remoteKey string)
 }
 
 // ConnConfig is a peer Connection configuration
@@ -705,6 +710,10 @@ func (conn *Conn) onWGDisconnected(watcherCtx context.Context) {
 
 	conn.Log.Warnf("WireGuard handshake timeout detected, closing current connection")
 
+	if conn.config.PQ != nil {
+		conn.config.PQ.OnDataPathDown(conn.config.Key)
+	}
+
 	// Close the active connection based on current priority
 	switch conn.currentConnPriority {
 	case conntype.Relay:
@@ -970,6 +979,11 @@ func (conn *Conn) onWGCheckSuccess() {
 	conn.mu.Lock()
 	conn.wgTimeouts = 0
 	conn.mu.Unlock()
+
+	// A fresh WireGuard handshake is the clock for the post-quantum PSK rotation.
+	if conn.config.PQ != nil {
+		conn.config.PQ.OnDataPathRekeyed(conn.config.Key)
+	}
 }
 
 // recordConnectionMetrics records connection stage timestamps as metrics

client/internal/pqkem_adapter.go

@@ -90,3 +90,13 @@ func (p pqHandshaker) SetRemotePort(remoteKey string, overlayIP netip.Addr, port
 	}
 	p.mgr.AddPeer(pqkem.RemoteID(remoteKey), netip.AddrPortFrom(overlayIP, uint16(port)))
 }
+
+// OnDataPathRekeyed clocks the next chained PSK rotation on a fresh WG handshake.
+func (p pqHandshaker) OnDataPathRekeyed(remoteKey string) {
+	p.mgr.OnDataPathRekeyed(pqkem.RemoteID(remoteKey))
+}
+
+// OnDataPathDown signals the peer's tunnel went down.
+func (p pqHandshaker) OnDataPathDown(remoteKey string) {
+	p.mgr.OnDataPathDown(pqkem.RemoteID(remoteKey))
+}