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/Models/Models.swift

@@ -131,6 +131,39 @@ final class Asset {
     }
 }
 
+@Model
+final class Symptom {
+    var name: String
+    var startedAt: Date
+    var endedAt: Date?
+    var note: String?
+    var severity: Int
+    var tags: [String]
+    var createdAt: Date
+
+    init(name: String,
+         startedAt: Date = .now,
+         endedAt: Date? = nil,
+         note: String? = nil,
+         severity: Int = 3,
+         tags: [String] = [],
+         createdAt: Date = .now) {
+        self.name = name
+        self.startedAt = startedAt
+        self.endedAt = endedAt
+        self.note = note
+        self.severity = max(1, min(5, severity))
+        self.tags = tags
+        self.createdAt = createdAt
+    }
+
+    var isOngoing: Bool { endedAt == nil }
+
+    var duration: TimeInterval {
+        (endedAt ?? .now).timeIntervalSince(startedAt)
+    }
+}
+
 @Model
 final class ChatTurn {
     var question: String