fix(web): 回答从开头开始读，两个会话面同一套钉顶跟随

frontend/src/components/rectification-agentic-chat.tsx

@@ -464,8 +464,7 @@ export function RectificationAgenticChat(props: RectificationAgenticChatProps) {
   const boardId = useId();
   const boardTitleId = useId();
   const composerRemainingId = useId();
-  // The same anchor-and-follow the consultation surface uses: streamed tokens and
-  // new cards land the viewport on the bottom only while the reader is there.
+  // Same pin-the-turn-head hook as the consultation surface (BUG-930).
   const conversationAnchor = useConversationScrollAnchor(conversation, true, caseId);
 
   useLayoutEffect(() => {
@@ -717,6 +716,7 @@ export function RectificationAgenticChat(props: RectificationAgenticChatProps) {
           : []),
         liveRow,
       ]));
+    if (!continuation) conversationAnchor.pinLatestTurn();
     let raw = "";
     let runOutcome: "succeeded" | "stopped" | "failed" = "failed";
     let activityTrace: readonly AgentActivityTraceItem[] = emptyActivityTrace();
@@ -1079,6 +1079,7 @@ export function RectificationAgenticChat(props: RectificationAgenticChatProps) {
         },
       },
     ]);
+    conversationAnchor.pinLatestTurn();
     setPending(true);
     const abortController = new AbortController();
     runAbort.current = abortController;
@@ -1231,6 +1232,7 @@ export function RectificationAgenticChat(props: RectificationAgenticChatProps) {
         activity: { phase: "evidence-validation", label: adoptingLabel, startedAt: Date.now() },
       },
     ]);
+    conversationAnchor.pinLatestTurn();
     const abortController = new AbortController();
     runAbort.current = abortController;
     try {
@@ -1812,7 +1814,7 @@ export function RectificationAgenticChat(props: RectificationAgenticChatProps) {
       </section>
 
         <div className="composer-wrap">
-          {!conversationAnchor.anchored && (
+          {conversationAnchor.latestBelowFold && (
             <JumpToLatestButton onClick={conversationAnchor.anchorToLatest} />
           )}
           <ChatComposer