feat: anchored-time infrastructure with a save-start anchor in idx v5

WriteAnchoredTime stores the absolute value; ReadAnchoredTime re-bases it
by the elapsed time since the loaded save started, so downtime does not
age it and an unchanged value serializes to identical bytes. Min/MaxValue
pass through unshifted and shifts saturate instead of overflowing. The
shift rides the reader instance, so parallel per-persistence loads and
single-file restores each see their own file's anchor.

World.SaveStartTime is stamped the moment the world freezes for a
snapshot - one anchor for the whole save - and written into the idx v5
header. Loading an idx v5 computes the shift and applies it to that
persistence's bin readers; v4 and older carry no anchor and read with a
zero shift, which existing hand-written-header tests already pin.

Nothing serializes anchored values yet; field conversions follow
separately. Covered by unit round-trip tests and an end-to-end
persistence test that re-bases across a simulated two-hour downtime.

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

Projects/Server/Serialization/BufferWriter.cs

@@ -407,6 +407,21 @@ public class BufferWriter : IGenericWriter
         Write(value.Ticks - DateTime.UtcNow.Ticks);
     }
 
+    /// <summary>
+    /// Writes the absolute value; <see cref="IGenericReader.ReadAnchoredTime" /> re-bases it
+    /// by the elapsed time since the save started, so downtime does not age it and an
+    /// unchanged value serializes to identical bytes.
+    /// </summary>
+    public void WriteAnchoredTime(DateTime value)
+    {
+        if (value.Kind == DateTimeKind.Local)
+        {
+            value = value.ToUniversalTime();
+        }
+
+        Write(value.Ticks);
+    }
+
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public void Write(IPAddress value)
     {