fix: 状态指示器重叠与 Trace Viewer 状态丢失

1. StreamingIndicator: 最后一条是 assistant 消息时不显示'正在连接', 避免与 AssistantMessage 的'正在思考'重叠

2. useAgentStream: 跳过 INIT(iteration=0) 的 trace step, 引擎初始化无实际内容不应展示

3. useAgentStream: 状态转换追加到 states 数组而非覆盖 state 字段, 保留完整 THINKING->EXECUTING->OBSERVING 进度

4. TraceStep: header 显示状态进度链(如'思考 -> 执行 -> 观察')而非仅最终状态

5. agent-store: 旧数据兼容, 加载时为缺少 states 的 trace step 补全

src/components/trace/TraceStep.tsx

@@ -64,13 +64,18 @@ export function TraceStep({ step, isCurrent }: TraceStepProps): React.JSX.Elemen
   const label = TRACE_STATE_LABELS[step.state] ?? step.state;
   const duration = step.completedAt ? step.completedAt - step.startedAt : null;
 
+  // 构建状态进度链（如 "思考 → 执行 → 观察"）
+  const stateChain = (step.states ?? [step.state])
+    .map((s) => TRACE_STATE_LABELS[s] ?? s)
+    .join(' → ');
+
   return (
     <Box sx={{ borderRadius: 1, border: '1px solid', borderColor: 'divider', bgcolor: 'secondary.main', transition: 'all 150ms', ...(isCurrent ? { boxShadow: `0 0 0 1px ${color}` } : {}) }}>
       <IconButton size="small" onClick={handleToggle} sx={{ width: '100%', justifyContent: 'flex-start', gap: 1, px: 1.5, py: 1, borderRadius: '4px 4px 0 0', color: 'text.primary', fontSize: 12 }}>
         {expanded ? <ChevronDown size={10} /> : <ChevronRight size={10} />}
         {isCurrent ? <Loader2 size={12} style={{ color, animation: 'spin 1s linear infinite' }} /> : step.completedAt ? <CheckCircle size={12} style={{ color }} /> : <CircleDot size={12} style={{ color }} />}
         <Typography component="span" sx={{ fontWeight: 600, color, fontSize: 12 }}>#{step.iteration}</Typography>
-        <Typography component="span" sx={{ textTransform: 'uppercase', color: 'text.secondary', fontSize: 12 }}>{label}</Typography>
+        <Typography component="span" sx={{ color: 'text.secondary', fontSize: 12 }}>{stateChain}</Typography>
         {duration != null && <Typography variant="caption" sx={{ ml: 'auto', color: 'text.secondary' }}>{formatDuration(duration)}</Typography>}
         {step.tokenUsage && <Typography variant="caption" sx={{ color: 'text.secondary' }}>{formatTokens(step.tokenUsage.totalTokens)} tok</Typography>}
       </IconButton>