refactor(terminology): 层级总结为主路径，synopsis 标记为旧式全局概要

- README: 明确 summaryState 为主总结体系，legacy synopsis 为兼容兜底
- prompt-profiles: synopsis 任务标签改为「小总结」
- schema/task-graph-stats/panel: synopsis 节点显示为「全局概要（旧）」
- index.js: fallback 状态文案改为「旧式全局概要生成/更新中」
- p0-regressions: 同步更新断言字符串

maintenance/task-graph-stats.js

@@ -8,7 +8,7 @@ const DEFAULT_TYPE_LABELS = Object.freeze({
   location: "地点",
   rule: "规则",
   thread: "主线",
-  synopsis: "全局概要",
+  synopsis: "全局概要（旧）",
   reflection: "反思",
   pov_memory: "主观记忆",
 });