Merge branch 'phase/73-lesson-marks'

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

src/HSchool.Simulation/School.cs

@@ -18,6 +18,7 @@ public sealed class School : IDisposable
     private bool _disposed;
     private readonly List<PersonLogEvent> _dayLog = [];
     private readonly HashSet<string> _lessonLogOnce = new(StringComparer.Ordinal);
+    private readonly HashSet<string> _lessonMarkOnce = new(StringComparer.Ordinal);
     private readonly List<WorldEvent> _worldEvents = [];
 
     internal School(int id, string name, DateTime startDate, DefCatalog? catalog, MapLayout? map)
@@ -160,6 +161,7 @@ public sealed class School : IDisposable
         _dayLog.Clear();
         LoggedActivity.Clear();
         _lessonLogOnce.Clear();
+        _lessonMarkOnce.Clear();
     }
 
     internal void AppendDayLog(PersonLogEvent row) => _dayLog.Add(row);
@@ -179,6 +181,15 @@ public sealed class School : IDisposable
         return true;
     }
 
+    /// <summary>
+    /// One lesson mark per person per calendar day and period. Survives teacher arriving mid-slot.
+    /// </summary>
+    internal bool TryClaimLessonMark(string personId, DateTime day, int period)
+    {
+        var key = string.Concat(personId, "\0", day.ToString("yyyy-MM-dd"), "\0", period.ToString());
+        return _lessonMarkOnce.Add(key);
+    }
+
     public void QueueDecision(string personId)
     {
         ObjectDisposedException.ThrowIf(_disposed, this);