feat(web): 流式帧按会话过滤，非激活会话不再串入当前气泡

- WsDownEvent 增 sessionId；流式分支（message/thinking/tool_call/tool_execution/stream_chunk/stream_end 及 legacy thinking/tool_used）统一过 streamFrameApplies
- 会话列表未解析(activeSessionId 为空)时保持改前行为，避免误丢主人自己的流
- 抽出 lib/streamScope.ts 承载该判断，ws.ts 回到 500 行红线内
- messageSync.test.ts 增 21 项断言（跨会话丢弃/激活追加/无 session 兼容/未解析回退）

frontend/web/src/lib/ws.ts

@@ -14,6 +14,7 @@ import {
   toRecord,
 } from "./wsParse";
 import { MODES, type AwakenState, type ChatMessage, type Mode, type WsUpEvent } from "./types";
+import { streamFrameApplies } from "./streamScope";
 
 /**
  * Realtime client for `/ws/chat`. Reconnects with exponential backoff and
@@ -83,10 +84,15 @@ export function dispatchFrame(ev: { type: string; sessionId?: string; data: unkn
 
   switch (ev.type) {
     case "stream_chunk": {
+      if (!streamFrameApplies(ev, r, s.activeSessionId)) break;
       if (r && typeof r.delta === "string") s.appendChunk(r.delta);
       break;
     }
     case "stream_end": {
+      // Another session's turn ending must not close the streaming bubble on
+      // screen or clear this conversation's busy badge (both are single-slot
+      // state: the last message's flag, and one shell-wide `sessionBusy`).
+      if (!streamFrameApplies(ev, r, s.activeSessionId)) break;
       s.endStream();
       // The assistant reply stopped streaming, so the immediate turn is over:
       // close the busy/pending window so the badge does not stick. If the backend
@@ -104,6 +110,7 @@ export function dispatchFrame(ev: { type: string; sessionId?: string; data: unkn
       break;
     }
     case "message.delta": {
+      if (!streamFrameApplies(ev, r, s.activeSessionId)) break;
       if (r && typeof r.delta === "string") s.appendChunk(r.delta);
       break;
     }
@@ -167,10 +174,14 @@ export function dispatchFrame(ev: { type: string; sessionId?: string; data: unkn
       break;
     }
     case "thinking.start": {
+      // Guarded like the deltas: only the active conversation may reset the
+      // accumulator (which also bounds a switched-away session's stale text).
+      if (!streamFrameApplies(ev, r, s.activeSessionId)) break;
       pendingThinking = "";
       break;
     }
     case "thinking.delta": {
+      if (!streamFrameApplies(ev, r, s.activeSessionId)) break;
       if (r && typeof r.delta === "string") {
         pendingThinking += r.delta;
         s.setThinking(pendingThinking);
@@ -178,12 +189,14 @@ export function dispatchFrame(ev: { type: string; sessionId?: string; data: unkn
       break;
     }
     case "thinking.end": {
+      if (!streamFrameApplies(ev, r, s.activeSessionId)) break;
       if (pendingThinking) s.setThinking(pendingThinking);
       pendingThinking = "";
       break;
     }
     case "tool_call.start":
     case "tool_call.end": {
+      if (!streamFrameApplies(ev, r, s.activeSessionId)) break;
       const tool = parseToolCall(ev.data);
       if (tool) s.addTool(tool);
       break;
@@ -194,11 +207,15 @@ export function dispatchFrame(ev: { type: string; sessionId?: string; data: unkn
       break;
     }
     case "tool_execution.update": {
+      // Same card as tool_call.* — same guard.
+      if (!streamFrameApplies(ev, r, s.activeSessionId)) break;
       const tool = parseToolCall(ev.data);
       if (tool) s.addTool(tool);
       break;
     }
     case "thinking": {
+      // Legacy single-shot CoT frame — renders into the same bubble.
+      if (!streamFrameApplies(ev, r, s.activeSessionId)) break;
       if (typeof ev.data === "string") {
         s.setThinking(ev.data);
       } else if (r && typeof r.text === "string") {
@@ -212,6 +229,8 @@ export function dispatchFrame(ev: { type: string; sessionId?: string; data: unkn
       break;
     }
     case "tool_used": {
+      // Legacy tool echo — same card as tool_call.*.
+      if (!streamFrameApplies(ev, r, s.activeSessionId)) break;
       const tool = parseTool(ev.data);
       if (tool) s.addTool(tool);
       break;