Keep conversations ordered while background work updates independently

Why: Repeated progress and coupled scrolling obscured chat, and task choices lacked a reliable independent interaction path.

Evidence: 285 frontend unit tests and 23 browser scenarios pass locally before the shared event-loader extraction; final combination checks follow in Dev.

Impact: Coalesce cumulative progress, retain execution records, show timestamps and quiet notifications, support versioned work actions and explicit result references without automatic chat backfill.

Refs: feature/workbench-flow-completion

src/scrollsync.ts

@@ -1,24 +1,4 @@
-// 中栏 → 右栏单向滚动同步，以及互定位后的 1.2s 闪烁。
-// 比例按可滚动范围计算；任一侧没有溢出时不同步，避免把静止栏拽动。
-
-export interface ScrollBox {
-  scrollTop: number
-  scrollHeight: number
-  clientHeight: number
-}
-
-export function scrollRatio(box: ScrollBox): number | null {
-  const range = box.scrollHeight - box.clientHeight
-  if (range <= 0) return null
-  return box.scrollTop / range
-}
-
-export function scrollTopForRatio(box: Pick<ScrollBox, 'scrollHeight' | 'clientHeight'>, ratio: number): number | null {
-  const range = box.scrollHeight - box.clientHeight
-  if (range <= 0) return null
-  const clamped = Math.min(1, Math.max(0, ratio))
-  return clamped * range
-}
+// Explicit message/card location feedback. The middle and work panes scroll independently.
 
 /** 给已定位的节点加上 mk-flash，1.2s 后摘掉。 */
 export function flashElement(el: Element | null) {