feat: convert all delta-time serialization to anchored time

Every actively-written delta-time value now stores an absolute anchored
timestamp, shifted by downtime at load: remaining time is preserved across
restarts and idle saves become byte-stable.

- Item v11: LastMoved (was whole-minute delta, rewritten every save) and
  DecayResetTime; Mobile v38: the three stat-gain stamps; BaseCreature v21:
  SummonEnd; plus VendorInventory, StealableArtifacts, and ML quest objective
  timers in their own version gates.
- 17 code-generated classes swap [DeltaDateTime] for [AnchoredDateTime] with a
  version bump and MigrateFrom each (generated from their current migration
  schemas, so old saves replay the delta reads).
- GenericPersistence payloads carry no anchor of their own, so they inherit
  the save-wide shift: entity indexes always load first, and every file in a
  save shares one SaveStartTime - World.LoadTimeShift is stamped from the idx
  v5 header and applied to generic persistence readers. This is what makes
  VirtueContext and StealableArtifacts convertible at all.
- Legacy read sites (old-version fallbacks and migration replays) keep
  ReadDeltaTime: they decode old bytes and must never change.

Acceptance tests: item serialization is byte-identical across saves hours
apart, and LastMoved/DecayResetTime round-trip exactly at sub-minute
precision (impossible under the old minutes encoding).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Projects/Server/Serialization/GenericPersistence.cs

@@ -98,7 +98,13 @@ public abstract class GenericPersistence : Persistence, IGenericSerializable
 
             byte* ptr = null;
             accessor.SafeMemoryMappedViewHandle.AcquirePointer(ref ptr);
-            var dataReader = new UnmanagedDataReader(ptr, accessor.Length, typesDb);
+            var dataReader = new UnmanagedDataReader(ptr, accessor.Length, typesDb)
+            {
+                // These payloads carry no anchor of their own; they inherit the save-wide
+                // shift stamped while the entity indexes were read (indexes always load
+                // before persistence payloads — see Persistence.Load).
+                AnchoredTimeShift = World.LoadTimeShift
+            };
             Deserialize(dataReader);
 
             error = dataReader.Position != fileLength