Refuse newer save formats and add a gated dump of a live school.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/HSchool.Server/Game/SchoolStore.cs

@@ -142,6 +142,21 @@ internal sealed class SchoolStore
                     continue;
                 }
 
+                if (save.Format > CurrentFormat)
+                {
+                    _logger.LogWarning(
+                        "Save {Path} is format {Format}; this build reads format {Current}. Leaving the file in place.",
+                        path,
+                        save.Format,
+                        CurrentFormat);
+                    continue;
+                }
+
+                if (save.Format < CurrentFormat)
+                {
+                    save = UpgradeOlderSave(save);
+                }
+
                 // Claimed last, so a file rejected above does not reserve an id a good file needs.
                 if (!claimed.TryAdd(save.Id, path))
                 {
@@ -178,6 +193,13 @@ internal sealed class SchoolStore
         return saves;
     }
 
+    /// <summary>
+    /// Named upgrade seam for saves older than <see cref="CurrentFormat"/>. Empty on purpose:
+    /// missing fields already default and extra fields are ignored. Put a migration here when a
+    /// format bump actually needs one.
+    /// </summary>
+    internal static SchoolSave UpgradeOlderSave(SchoolSave save) => save;
+
     public void Save(SchoolSave save)
     {
         WriteAtomic(SchoolPath(save.Id), save);