fix: LastMoved and stat-gain stamps stay absolute across restarts (#2729)

ReadAnchoredTime shifted Item.LastMoved, Item.DecayResetTime and
Mobile.LastStr/Int/DexGain forward by the downtime at load. The bytes
were stable from save to save, but the first save after every restart
rewrote one timestamp in nearly every item record and most mobile
records, which defeats any per-record backup or delta save.

- LastMoved and DecayResetTime are read as saved. The downtime credit
  is applied after load by Item.CreditDowntimeToDecay, which moves
  DecayResetTime only for items whose decay countdown was still running
  when the save started. Every other item keeps its bytes, and decay
  timing is unchanged.
- World.LoadedSaveStartTime exposes the loaded save's anchor so the
  credit can tell which countdowns were running.
- Stat-gain stamps are read raw. The cooldown is minutes long, so
  downtime counting toward it costs nothing.

No format change: these fields were already absolute ticks.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Projects/Server/Items/Item.cs

@@ -1042,13 +1042,14 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
 
         writer.Write((int)flags);
 
-        // Anchored: shifted by downtime at load, so time-since-moved is preserved and the
-        // bytes are stable across saves while the item does not move.
-        writer.WriteAnchoredTime(LastMoved);
+        // Absolute and never shifted at load, so an item that does not move serializes to the
+        // same bytes across saves and across restarts. Downtime credit for a decay countdown that
+        // was running at save time goes into DecayResetTime (CreditDowntimeToDecay).
+        writer.Write(LastMoved);
 
         if (GetSaveFlag(flags, SaveFlag.DecayReset))
         {
-            writer.WriteAnchoredTime(info.m_DecayReset);
+            writer.Write(info.m_DecayReset);
         }
 
         if (GetSaveFlag(flags, SaveFlag.Direction))
@@ -2494,6 +2495,30 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
         UpdateDecayRegistration();
     }
 
+    /// <summary>
+    /// Credits the downtime before this load to a decay countdown that was still running when the
+    /// save started, so the shard being down does not age the item. <see cref="LastMoved" /> stays
+    /// the real time of the move; the credit goes into <see cref="DecayResetTime" />, which only
+    /// the items moved within their <see cref="DecayTime" /> of the save ever carry. Called once
+    /// per item after the world loads, when <see cref="DecayTime" /> is final. An item already
+    /// due at save time gets nothing: it is due either way.
+    /// </summary>
+    internal void CreditDowntimeToDecay(DateTime saveStartUtc, TimeSpan downtime)
+    {
+        if (downtime <= TimeSpan.Zero || !Decays)
+        {
+            return;
+        }
+
+        var reset = DecayResetTime;
+        var start = reset > LastMoved ? reset : LastMoved;
+
+        if (start + DecayTime > saveStartUtc)
+        {
+            DecayResetTime = start + downtime;
+        }
+    }
+
     public void UpdateDecayRegistration()
     {
         DecayScheduler.Unregister(this);
@@ -2856,7 +2881,9 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
 
                     if (version >= 11)
                     {
-                        LastMoved = reader.ReadAnchoredTime();
+                        // Written with WriteAnchoredTime before LastMoved stopped being shifted;
+                        // the bytes are the same absolute ticks either way.
+                        LastMoved = reader.ReadDateTime();
                     }
                     else if (version < 7)
                     {
@@ -2878,7 +2905,7 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
 
                     if (version >= 10 && GetSaveFlag(flags, SaveFlag.DecayReset))
                     {
-                        var reset = version >= 11 ? reader.ReadAnchoredTime() : reader.ReadDeltaTime();
+                        var reset = version >= 11 ? reader.ReadDateTime() : reader.ReadDeltaTime();
 
                         // Pre-v11 LastMoved was stored at whole-minute precision; keep the
                         // stamp only while it still extends the deadline.

Projects/Server/Mobiles/Mobile.cs

@@ -2343,9 +2343,11 @@ public partial class Mobile : IHued, IComparable<Mobile>, ISpawnable, IObjectPro
     {
         writer.Write(38); // version
 
-        writer.WriteAnchoredTime(LastStrGain);
-        writer.WriteAnchoredTime(LastIntGain);
-        writer.WriteAnchoredTime(LastDexGain);
+        // Absolute and never shifted at load: the stat-gain cooldown is minutes long, so letting
+        // downtime count toward it costs nothing, and an idle mobile keeps its bytes across restarts.
+        writer.Write(LastStrGain);
+        writer.Write(LastIntGain);
+        writer.Write(LastDexGain);
 
         byte hairflag = 0x00;
 
@@ -6179,9 +6181,9 @@ public partial class Mobile : IHued, IComparable<Mobile>, ISpawnable, IObjectPro
                 {
                     if (version >= 38)
                     {
-                        LastStrGain = reader.ReadAnchoredTime();
-                        LastIntGain = reader.ReadAnchoredTime();
-                        LastDexGain = reader.ReadAnchoredTime();
+                        LastStrGain = reader.ReadDateTime();
+                        LastIntGain = reader.ReadDateTime();
+                        LastDexGain = reader.ReadDateTime();
                     }
                     else
                     {

Projects/Server/Serialization/GenericEntityPersistence.cs

@@ -529,6 +529,7 @@ public class GenericEntityPersistence<T> : GenericPersistence, IGenericEntityPer
             // The whole save shares one anchor. Publish it so payloads without their own
             // (GenericPersistence bins) can shift too; indexes load before any of them.
             World.LoadTimeShift = _anchoredTimeShift;
+            World.LoadedSaveStartTime = anchor;
         }
 
         if (version >= 4)

Projects/Server/World/World.cs

@@ -108,6 +108,12 @@ public static class World
     /// payloads, which carry no anchor of their own. Zero for saves that predate the anchor.
     /// </summary>
     public static TimeSpan LoadTimeShift { get; internal set; }
+
+    /// <summary>
+    /// UTC start time of the save being loaded, read from its index anchor. Default for saves
+    /// that predate the anchor.
+    /// </summary>
+    public static DateTime LoadedSaveStartTime { get; internal set; }
     public static bool Running => WorldState is not WorldState.Loading and not WorldState.Initial;
     public static bool Loading => WorldState == WorldState.Loading;
 
@@ -674,6 +680,7 @@ public static class World
                 }
 
                 item.ClearProperties();
+                item.CreditDowntimeToDecay(LoadedSaveStartTime, LoadTimeShift);
                 item.UpdateDecayRegistration();
             }
         }