feat: conversation state machine, mic meter, history, wake lock (Phase 3)

Sources/LiveKit/TranscriptHistory.swift

@@ -0,0 +1,43 @@
+import Foundation
+
+struct HistoryEntry: Codable, Equatable {
+    let role: String
+    let text: String
+    let timestamp: Date
+}
+
+/// Last 50 final transcript entries persisted as JSON in UserDefaults.
+@MainActor
+final class TranscriptHistory: ObservableObject {
+    @Published private(set) var entries: [HistoryEntry] = []
+
+    private static let key = "voice_history"
+    private static let maxEntries = 50
+
+    init() {
+        if let raw = UserDefaults.standard.string(forKey: Self.key),
+           let data = raw.data(using: .utf8),
+           let decoded = try? JSONDecoder().decode([HistoryEntry].self, from: data) {
+            entries = Array(decoded.suffix(Self.maxEntries))
+        }
+    }
+
+    func add(role: String, text: String) {
+        guard !text.isEmpty else { return }
+        entries.append(HistoryEntry(role: role, text: text, timestamp: Date()))
+        if entries.count > Self.maxEntries {
+            entries.removeFirst(entries.count - Self.maxEntries)
+        }
+        persist()
+    }
+
+    func clear() {
+        entries.removeAll()
+        persist()
+    }
+
+    private func persist() {
+        guard let data = try? JSONEncoder().encode(entries) else { return }
+        UserDefaults.standard.set(String(decoding: data, as: UTF8.self), forKey: Self.key)
+    }
+}