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/adapter.go

@@ -34,8 +34,11 @@ type Adapter interface {
 	Name() string
 	// Chat performs a non-streaming completion and returns the full response.
 	Chat(ctx context.Context, req Request, opts Options) (Response, error)
-	// ChatStream streams token-level deltas via onChunk and returns the
-	// aggregated response (with usage when the provider reports it). onChunk may
-	// be nil to discard deltas while still aggregating.
-	ChatStream(ctx context.Context, req Request, onChunk func(delta string)) (Response, error)
+	// ChatStream streams structured events via onEvent and returns the aggregated
+	// response (with usage when the provider reports it). onEvent may be nil to
+	// discard events while still aggregating. Every provider-supported segment is
+	// bracketed (text_start/delta/end, tool_call_start/delta/end), so the consumer
+	// can render each independently; segments adapters cannot yet distinguish are
+	// delivered as text_delta events.
+	ChatStream(ctx context.Context, req Request, onEvent func(StreamEvent)) (Response, error)
 }