feat(symptom): add Symptom @Model + start/end sheets + ongoing card

- Symptom @Model with severity 1-5 clamp, isOngoing, duration helpers
- SymptomStartSheet / SymptomEndSheet / OngoingSymptomsCard
- RecordSheet 加 .symptom kind 入口
- RootView 增加 'records' tab + ArchiveListView placeholder
- HomeView 顶部加 OngoingSymptomsCard
- ModelsSchemaTests: 2 个 Symptom 烟测(ongoing predicate + severity clamp)

Note: Symptom 是 CLAUDE.md §10 清单外的新功能,由产品负责人决定加入。
ArchiveListView 仍是 placeholder,真实 C1 实现按计划在 W4。

康康/Features/Home/HomeView.swift

@@ -10,6 +10,9 @@ struct HomeView: View {
                     .padding(.top, 4)
                     .padding(.bottom, 18)
 
+                OngoingSymptomsCard()
+                    .padding(.bottom, 18)
+
                 todaySummaryCard
                     .padding(.bottom, 18)