Added the system from trail

Runtime/QuestLog.cs

@@ -0,0 +1,80 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Jovian.EncounterSystem {
+    /// <summary>Tag for a quest log entry.</summary>
+    public enum QuestLogEventType {
+        Started,
+        Advanced,
+        Completed
+    }
+
+    /// <summary>
+    /// One chronological entry in the quest log. Names are cached alongside ids so a loaded save
+    /// can display the log even if the underlying encounter has since been renamed or removed.
+    /// </summary>
+    [Serializable]
+    public class QuestLogEntry {
+        public QuestLogEventType type;
+        public string encounterInternalId;
+        public string encounterName;
+        public string fromEncounterName;
+    }
+
+    /// <summary>
+    /// Chronological, serializable record of quest events. Subscribes to <see cref="QuestProgress"/>
+    /// events at construction time — build it before any encounter fires so nothing is missed.
+    /// </summary>
+    /// <remarks>
+    /// This is the save payload for quest progression. On load, call <see cref="Restore"/> with the
+    /// saved entries and then <see cref="QuestProgress.LoadResolvedIds"/> with
+    /// <see cref="ResolvedEncounterIds"/> to rehydrate the gating set.
+    /// </remarks>
+    public class QuestLog {
+        private readonly List<QuestLogEntry> entries = new();
+
+        /// <summary>The log in chronological order.</summary>
+        public IReadOnlyList<QuestLogEntry> Entries => entries;
+
+        /// <summary>Subscribe to <paramref name="progress"/>'s quest events; every fire appends an entry.</summary>
+        public QuestLog(QuestProgress progress) {
+            progress.QuestStarted += quest => Record(QuestLogEventType.Started, null, quest);
+            progress.QuestAdvanced += (from, to) => Record(QuestLogEventType.Advanced, from, to);
+            progress.QuestCompleted += quest => Record(QuestLogEventType.Completed, null, quest);
+        }
+
+        /// <summary>Return a copy of the current entries suitable for serialization.</summary>
+        public List<QuestLogEntry> CreateSnapshot() {
+            return new List<QuestLogEntry>(entries);
+        }
+
+        /// <summary>Replace the current entries with those from a save. Pass the list straight from
+        /// the save payload.</summary>
+        public void Restore(IEnumerable<QuestLogEntry> saved) {
+            entries.Clear();
+            if(saved == null) {
+                return;
+            }
+
+            entries.AddRange(saved);
+        }
+
+        /// <summary>Enumerate the distinct encounter ids present in the log — what
+        /// <see cref="QuestProgress.LoadResolvedIds"/> needs on load.</summary>
+        public IEnumerable<string> ResolvedEncounterIds() {
+            return entries
+                .Select(entry => entry.encounterInternalId)
+                .Where(id => !string.IsNullOrEmpty(id));
+        }
+
+        private void Record(QuestLogEventType type, IEncounter from, IEncounter to) {
+            entries.Add(new QuestLogEntry {
+                type = type,
+                encounterInternalId = to?.EncounterDefinition?.internalId,
+                encounterName = to?.EncounterDefinition?.name,
+                fromEncounterName = from?.EncounterDefinition?.name
+            });
+        }
+    }
+}