feat: mark WriteDeltaTime obsolete; document ReadDeltaTime as legacy-decode-only

WriteDeltaTime has no remaining callers - with warnings-as-errors, any new
delta-time write now fails the build with migration instructions in the
message. ReadDeltaTime stays un-attributed: its ~25 remaining callers (old-
version fallbacks and generated migration replays) decode existing bytes and
are correct forever; its docs now say exactly that.

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

Projects/Server/Serialization/BufferWriter.cs

@@ -384,6 +384,7 @@ public class BufferWriter : IGenericWriter
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    [Obsolete("Delta time rewrites its bytes on every save. Write anchored time instead (WriteAnchoredTime, or [AnchoredDateTime] on generated fields); bump the containing type's version, as the wire format changes. Existing delta payloads remain readable through ReadDeltaTime in old-version fallbacks.")]
     public void WriteDeltaTime(DateTime value)
     {
         if (value == DateTime.MinValue)

Projects/Server/Serialization/IGenericReader.cs

@@ -43,6 +43,12 @@ public interface IGenericReader
     DateTime ReadDateTime() => new(ReadLong(), DateTimeKind.Utc);
     TimeSpan ReadTimeSpan() => new(ReadLong());
 
+    /// <summary>
+    /// Decodes a legacy delta-time value. Only for reading old-version payloads (version
+    /// fallbacks and migration replays) — current formats store anchored time and read it
+    /// with <see cref="ReadAnchoredTime" />. <see cref="IGenericWriter.WriteDeltaTime" /> is
+    /// obsolete: no current-version format may write delta time.
+    /// </summary>
     DateTime ReadDeltaTime()
     {
         return ReadLong() switch

Projects/Server/Serialization/IGenericWriter.cs

@@ -40,7 +40,10 @@ public interface IGenericWriter
     void Write(decimal value);
     void WriteEncodedInt(int value);
     void Write(DateTime value);
+
+    [Obsolete("Delta time rewrites its bytes on every save. Write anchored time instead (WriteAnchoredTime, or [AnchoredDateTime] on generated fields); bump the containing type's version, as the wire format changes. Existing delta payloads remain readable through ReadDeltaTime in old-version fallbacks.")]
     void WriteDeltaTime(DateTime value);
+
     void WriteAnchoredTime(DateTime value);
     void Write(IPAddress value);
     void Write(TimeSpan value);