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/App/KangkangApp.swift

@@ -10,6 +10,7 @@ struct KangkangApp: App {
             DiaryEntry.self,
             Asset.self,
             ChatTurn.self,
+            Symptom.self,
         ])
         let config = ModelConfiguration(schema: schema, isStoredInMemoryOnly: false)
         do {