feat(model): Top5#1 结构化流事件 + WS 事件体系（对齐 OpenClaw AssistantMessageEvent）

- 新增 model/stream.go：StreamKind(text/thinking/tool_call 各 start/delta/end + tool_execution)
  + StreamEvent{Kind,Delta,Index,ID}。
- ChatStream 签名 onChunk func(delta string)→onEvent func(StreamEvent)；loop OnStream 同改；
  streamStep 转发结构化事件。
- openai：text_delta + text_start/text_end（正文三段式）；anthropic：文本块 text_*/tool_call_*
  按块 Index（content_block_stop 分支）真分段；fallback：有事件即交付。
- ws_hub：wsEvent 壳补 turnId/seq；新增 broadcastStream；streamChat 每帧打 turnId+seq；
  stream.go 把事件 publish 到 bus ws.stream。
- ws_bridge：streamEventData 映射 text_*→message.start/delta/end、thinking_*→thinking.*、
  tool_call_*→tool_call.*、tool_execution_update→tool_execution.update，未知回退 stream_chunk。
- 前端 ws.ts：解析 message.start/delta/end、thinking.start/delta/end(pendingThinking)、
  tool_call.start/end→addTool、tool_execution.update；保留旧 stream_chunk/thinking/tool_used 兼容；
  types.ts 补 turnId/seq。Docs/api-client.md 增 §3.1 v0.4。

已知限制（注释/文档标注）：turnId 在 OnStream→bus→relay 路径暂为空（StreamEvent 无 turnId 字段，
bus ws.stream 载荷不带 turn）；OpenAI adapter 未解析 tool_call 段；thinking 段两 adapter 暂不发；
tool_execution.update 是保留项（随工具循环钩子后置）。go build/vet/test + 前端 tsc 全绿。

internal/runtime/model/openai.go

@@ -194,9 +194,9 @@ func (a *OpenAIChatAdapter) Chat(ctx context.Context, req Request, opts Options)
 	return out, nil
 }
 
-// ChatStream streams token-level deltas via onChunk and returns the aggregated
+// ChatStream streams structured events via onEvent and returns the aggregated
 // response with usage when the provider reports it in the final chunk.
-func (a *OpenAIChatAdapter) ChatStream(ctx context.Context, req Request, onChunk func(delta string)) (Response, error) {
+func (a *OpenAIChatAdapter) ChatStream(ctx context.Context, req Request, onEvent func(StreamEvent)) (Response, error) {
 	start := time.Now()
 	if a.baseURL == "" {
 		return Response{}, ErrUpstream
@@ -222,7 +222,7 @@ func (a *OpenAIChatAdapter) ChatStream(ctx context.Context, req Request, onChunk
 	if resp.StatusCode != http.StatusOK {
 		return Response{}, a.upstreamError(resp)
 	}
-	out, err := scanSSE(resp.Body, onChunk)
+	out, err := scanSSE(resp.Body, onEvent)
 	if err != nil {
 		return Response{}, err
 	}
@@ -380,12 +380,14 @@ func responseFromWire(wire *openAIChatResponse) Response {
 
 // scanSSE incrementally decodes stream chunks, accumulates the full content
 // with a strings.Builder (never reading the whole body at once) and captures
-// usage from the final chunk.
-func scanSSE(r io.Reader, onChunk func(string)) (Response, error) {
+// usage from the final chunk. Each text delta is delivered as a text_delta
+// StreamEvent with the body-text segment bracketed by text_start/text_end.
+func scanSSE(r io.Reader, onEvent func(StreamEvent)) (Response, error) {
 	scanner := bufio.NewScanner(r)
 	scanner.Buffer(make([]byte, 0, sseBufInit), maxSSELineBytes)
 	agg := Response{}
 	var b strings.Builder
+	started := false
 	for scanner.Scan() {
 		line := strings.TrimSpace(scanner.Text())
 		if line == "" || !strings.HasPrefix(line, sseDataPrefix) {
@@ -412,13 +414,20 @@ func scanSSE(r io.Reader, onChunk func(string)) (Response, error) {
 			continue
 		}
 		b.WriteString(delta)
-		if onChunk != nil {
-			onChunk(delta)
+		if onEvent != nil {
+			if !started {
+				started = true
+				onEvent(StreamEvent{Kind: StreamKindTextStart})
+			}
+			onEvent(StreamEvent{Kind: StreamKindTextDelta, Delta: delta})
 		}
 	}
 	if err := scanner.Err(); err != nil {
 		return Response{}, fmt.Errorf("model: %w: read stream: %v", ErrUpstream, err)
 	}
+	if onEvent != nil && started {
+		onEvent(StreamEvent{Kind: StreamKindTextEnd})
+	}
 	agg.Content = b.String()
 	agg.Message = Message{Role: RoleAssistant, Content: []Content{{Type: ContentTypeText, Text: agg.Content}}}
 	return agg, nil