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

ui/panel.js

@@ -3163,7 +3163,7 @@ function _buildLegend() {
     { key: "location", label: "地点" },
     { key: "thread", label: "主线" },
     { key: "rule", label: "规则" },
-    { key: "synopsis", label: "概要" },
+    { key: "synopsis", label: "全局概要（旧）" },
     { key: "reflection", label: "反思" },
     { key: "pov_memory", label: "主观记忆" },
   ];
@@ -10783,7 +10783,7 @@ function _typeLabel(type) {
     location: "地点",
     thread: "主线",
     rule: "规则",
-    synopsis: "概要",
+    synopsis: "全局概要（旧）",
     reflection: "反思",
     pov_memory: "主观记忆",
   };