Add subgraph skeleton classes (#997)

Allows downstream consumers to use subgraph types ahead of impl.

src/subgraph/SubgraphOutput.ts

@@ -0,0 +1,23 @@
+import type { Point, ReadOnlyRect } from "@/interfaces"
+
+import { SubgraphSlot } from "./SubgraphSlotBase"
+
+export class SubgraphOutput extends SubgraphSlot {
+  get labelPos(): Point {
+    const [x, y, , height] = this.boundingRect
+    return [x + height, y + height * 0.5]
+  }
+
+  override arrange(rect: ReadOnlyRect): void {
+    const [left, top, width, height] = rect
+    const { boundingRect: b, pos } = this
+
+    b[0] = left
+    b[1] = top
+    b[2] = width
+    b[3] = height
+
+    pos[0] = left + height * 0.5
+    pos[1] = top + height * 0.5
+  }
+}