feat(verifier): decode a container-only scroll

Claude-Session: https://claude.ai/code/session_01A5KmftdEJ49A9z5mF5ESrX

internal/verifier/marshal.go

@@ -464,6 +464,7 @@ func DecodeAction(raw json.RawMessage) (Action, error) {
 	case "Scroll":
 		return Action{
 			Kind:           ActionKindScroll,
+			On:             wire.Selector,
 			Direction:      wire.Direction,
 			FromX:          wire.FromX,
 			FromY:          wire.FromY,

internal/verifier/marshal_test.go

@@ -47,6 +47,13 @@ func TestDecodeAction_AllKinds(t *testing.T) {
 			raw:  `{"kind":"Scroll","direction":"down","fromX":1,"fromY":2,"toX":3,"toY":4,"durationMillis":100}`,
 			want: Action{Kind: ActionKindScroll, Direction: "down", FromX: 1, FromY: 2, ToX: 3, ToY: 4, DurationMillis: 100},
 		},
+		{
+			// An author names the container and leaves the drag to the runner,
+			// which needs the selector to size it against the right bounds.
+			name: "Scroll with a container and no endpoints",
+			raw:  `{"kind":"Scroll","direction":"up","selector":"id:list","fromX":0,"fromY":0,"toX":0,"toY":0,"durationMillis":250}`,
+			want: Action{Kind: ActionKindScroll, On: "id:list", Direction: "up", DurationMillis: 250},
+		},
 		{
 			name: "PressKey",
 			raw:  `{"kind":"PressKey","key":"back"}`,