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/BinaryFileReader.cs

@@ -74,6 +74,12 @@ public sealed unsafe class BinaryFileReader : IDisposable, IGenericReader
     /// </summary>
     public long Position => _reader.Position;
 
+    public TimeSpan AnchoredTimeShift
+    {
+        get => _reader.AnchoredTimeShift;
+        set => _reader.AnchoredTimeShift = value;
+    }
+
     public void Dispose()
     {
         _accessor?.SafeMemoryMappedViewHandle.ReleasePointer();