Add rain and snow minutes to the morning commute.

Day plans take a precomputed extra; Ai does not know precipitation.

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

src/HSchool.Ai/DayPlan.cs

@@ -10,6 +10,10 @@ public readonly record struct DayPlan(DateOnly Day, DateTime? AppearAt, DateTime
     public bool Comes => AppearAt is not null;
 }
 
+/// <summary>
+/// Day plans. Rain and snow arrive as already-counted extra minutes — this project does not
+/// know precipitation.
+/// </summary>
 public static class DayPlans
 {
     public static DayPlan Build(
@@ -20,7 +24,8 @@ public static class DayPlans
         Timetable? timetable,
         DateTime time,
         int weekDays,
-        int schoolSeed)
+        int schoolSeed,
+        int extraCommuteMinutes = 0)
     {
         ArgumentNullException.ThrowIfNull(catalog);
         ArgumentNullException.ThrowIfNull(walks);
@@ -48,7 +53,7 @@ public static class DayPlans
             travel = 0f;
         }
 
-        var slack = SlackMinutes(catalog, person, schoolSeed, day);
+        var slack = SlackMinutes(catalog, person, schoolSeed, day) + Math.Max(0, extraCommuteMinutes);
         var appear = DateTime.SpecifyKind(utc.Date.Add(firstStart.ToTimeSpan()).AddMinutes(-(travel + slack)), DateTimeKind.Utc);
         var walkHome = DateTime.SpecifyKind(utc.Date.Add(lastEnd.ToTimeSpan()), DateTimeKind.Utc);
         return new DayPlan(day, appear, walkHome, firstRoom);