The mesh's interface takes over the found tunnel's MTU

Carries MTU from the reported tunnel (mesh-host#28) through inventory,
the overlay graph's TakeOver, into the generated config's [Interface].
A tuned path keeps its MTU across the takeover instead of regressing to
1420 and hanging transfers no ping would reveal. Two emit tests; a
tunnel with no MTU writes no line.

internal/link/enrolment.go

@@ -140,7 +140,7 @@ func (e Enrolment) Enrol(ctx context.Context, request EnrolRequest) (reply Enrol
 		}
 		if err := e.Inventory.RecordTunnel(ctx, node.ID, inventory.Tunnel{
 			Interface: request.Tunnel.Interface, Unit: request.Tunnel.Unit,
-			Config: request.Tunnel.Config, Port: request.Tunnel.Port,
+			Config: request.Tunnel.Config, Port: request.Tunnel.Port, MTU: request.Tunnel.MTU,
 			Address: request.Tunnel.Address, Range: request.Tunnel.Range,
 			PublicKey: request.Tunnel.PublicKey, Peers: peers,
 		}); err != nil {
@@ -199,7 +199,7 @@ func (e Enrolment) rekey(ctx context.Context, node inventory.Node, r Rekey) erro
 		peers = append(peers, inventory.TunnelPeer{PublicKey: p.PublicKey, Address: p.Address})
 	}
 	err := e.Inventory.Rekey(ctx, node.ID, r.Previous, r.OverlayKey, inventory.Tunnel{
-		Interface: r.Tunnel.Interface, Unit: r.Tunnel.Unit, Config: r.Tunnel.Config, Port: r.Tunnel.Port,
+		Interface: r.Tunnel.Interface, Unit: r.Tunnel.Unit, Config: r.Tunnel.Config, Port: r.Tunnel.Port, MTU: r.Tunnel.MTU,
 		Address: r.Tunnel.Address, Range: r.Tunnel.Range, PublicKey: r.Tunnel.PublicKey, Peers: peers,
 	})
 	if err != nil {

internal/link/protocol.go

@@ -93,6 +93,7 @@ type Tunnel struct {
 	Unit      string       `json:"unit"`
 	Config    string       `json:"config"`
 	Port      int          `json:"port"`
+	MTU       int          `json:"mtu,omitempty"`
 	Address   string       `json:"address"`
 	Range     string       `json:"range"`
 	PublicKey string       `json:"public_key"`