Add blank vars to ensure layouts satisfy tomo.Layout

layouts/contract.go

@@ -3,6 +3,8 @@ package layouts
 import "image"
 import "git.tebibyte.media/tomo/tomo"
 
+var _ tomo.Layout = ContractVertical
+
 // Contract is a layout that arranges boxes in a simple row or column according
 // to their minimum sizes.
 type Contract bool

layouts/cut.go

@@ -3,6 +3,8 @@ package layouts
 import "image"
 import "git.tebibyte.media/tomo/tomo"
 
+var _ tomo.Layout = new(cut)
+
 // Cut is a layout that can be divided into smaller and smaller sections.
 type Cut struct {
 	branches []*Cut

layouts/flow.go

@@ -3,6 +3,8 @@ package layouts
 import "image"
 import "git.tebibyte.media/tomo/tomo"
 
+var _ tomo.Layout = FlowVertical
+
 // Flow is a grid layout where the number of rows and columns changes depending
 // on the size of the container. It is designed to be used with an overflowing
 // container. If the container does not overflow in the correct direction, the

layouts/grid.go

@@ -4,6 +4,8 @@ import "math"
 import "image"
 import "git.tebibyte.media/tomo/tomo"
 
+var _ tomo.Layout = Grid { }
+
 // Grid is a layout that arranges boxes in a grid formation with distinct rows
 // and columns. It is great for creating forms.
 type Grid struct {