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/CalendarDefs.cs

@@ -16,6 +16,26 @@ public sealed class DayFrameDef : Def
     public int LongBreakAfter { get; init; }
 
     public int LongBreakMinutes { get; init; }
+
+    /// <summary>
+    /// Which break each parallel eats at. Empty means one sitting for the whole school, at
+    /// <see cref="LongBreakAfter"/>. Every listed break is a long one, so a school that feeds its
+    /// juniors and seniors separately gets two wide breaks rather than one crowded canteen.
+    /// </summary>
+    public IReadOnlyList<LunchBreakDef> LunchBreaks { get; init; } = [];
+}
+
+/// <summary>One lunch sitting: the break it happens at and the parallels it feeds.</summary>
+public sealed class LunchBreakDef
+{
+    /// <summary>The sitting fills the break after this 1-based lesson.</summary>
+    public int AfterLesson { get; init; }
+
+    public int GradeMin { get; init; }
+
+    public int GradeMax { get; init; }
+
+    public bool Covers(int year) => year >= GradeMin && year <= GradeMax;
 }
 
 public sealed class MonthDay