Enhance school day structure and decision-making for lunch breaks

- Updated `ai.md` to clarify the mechanics of hunger restoration and the importance of lunch breaks in the school schedule.
- Revised `schedule.md` to detail the new lunch break structure, allowing for separate sittings for different grade levels.
- Enhanced `Decision.cs` and `DecisionPlanner.cs` to incorporate logic for lunch breaks, ensuring that students only leave lessons during their designated lunch windows.
- Updated `DayFrameDef` and related classes to support multiple lunch breaks and validate their configurations.
- Adjusted tests to validate the new decision-making logic regarding lunch breaks and hunger management, ensuring robust functionality.
- Improved localization strings to reflect changes in the school day structure and lunch functionalities.

src/HSchool.Content/PeopleDefValidator.cs

@@ -391,6 +391,31 @@ internal static class PeopleDefValidator
         {
             throw new ContentLoadException($"DayFrameDef '{frame.DefName}' longBreakAfter must be 0 or a lesson before the last.");
         }
+
+        var fed = new HashSet<int>();
+        foreach (var sitting in frame.LunchBreaks)
+        {
+            if (sitting.AfterLesson < 1 || sitting.AfterLesson >= frame.LessonCount)
+            {
+                throw new ContentLoadException(
+                    $"DayFrameDef '{frame.DefName}' has a lunch break after lesson {sitting.AfterLesson}; it must be a lesson before the last.");
+            }
+
+            if (sitting.GradeMin < 1 || sitting.GradeMax < sitting.GradeMin)
+            {
+                throw new ContentLoadException(
+                    $"DayFrameDef '{frame.DefName}' has a lunch break with grades {sitting.GradeMin}–{sitting.GradeMax}.");
+            }
+
+            for (var year = sitting.GradeMin; year <= sitting.GradeMax; year++)
+            {
+                if (!fed.Add(year))
+                {
+                    throw new ContentLoadException(
+                        $"DayFrameDef '{frame.DefName}' feeds grade {year} at two lunch breaks.");
+                }
+            }
+        }
     }
 
     private static void ValidateAction(ActionDef action, DefCatalog catalog)