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/fallback.go

@@ -60,10 +60,10 @@ func (c *Chain) Chat(ctx context.Context, req Request, opts Options) (Response,
 }
 
 // ChatStream tries each step in order. If a step fails before delivering any
-// delta, the chain moves to the next step; once a step has delivered output it
+// event, the chain moves to the next step; once a step has delivered output it
 // is committed, so a mid-stream failure surfaces as an error rather than mixing
 // content across providers.
-func (c *Chain) ChatStream(ctx context.Context, req Request, onChunk func(delta string)) (Response, error) {
+func (c *Chain) ChatStream(ctx context.Context, req Request, onEvent func(StreamEvent)) (Response, error) {
 	if len(c.steps) == 0 {
 		return Response{}, ErrNoSteps
 	}
@@ -74,10 +74,10 @@ func (c *Chain) ChatStream(ctx context.Context, req Request, onChunk func(delta
 			stepReq.Model = s.Model
 		}
 		delivered := false
-		wrapped := func(delta string) {
+		wrapped := func(ev StreamEvent) {
 			delivered = true
-			if onChunk != nil {
-				onChunk(delta)
+			if onEvent != nil {
+				onEvent(ev)
 			}
 		}
 		resp, err := s.Adapter.ChatStream(ctx, stepReq, wrapped)