feat: drag relation tags along threads

src/types.ts

@@ -64,6 +64,10 @@ export interface Connection {
   /** Percentage from slack (0) to taut (100). */
   tightness?: number
   tagStyle?: 'luggage' | 'compact'
+  /** Percentage along the thread from its source exhibit. */
+  tagPosition?: number
+  /** Signed perpendicular distance from the thread, constrained by tightness. */
+  tagOffset?: number
 }
 
 export interface Viewport { x: number; y: number; zoom: number }