Add EventDef notices, morning and lesson bells, and info toasts.

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

src/HSchool.Content/EventDefs.cs

@@ -0,0 +1,38 @@
+namespace HSchool.Content;
+
+public static class EventSeverities
+{
+    public const string Info = "info";
+    public const string Warning = "warning";
+    public const string Error = "error";
+}
+
+public static class EventTriggers
+{
+    public const string DayStart = "dayStart";
+    public const string LessonStart = "lessonStart";
+    public const string GenerationFailed = "generationFailed";
+}
+
+public static class EventActions
+{
+    public const string None = "none";
+    public const string GenerateImage = "generateImage";
+}
+
+/// <summary>
+/// A fact the world can raise. Systems emit a trigger; the worker matches this def and builds a notice.
+/// </summary>
+public sealed class EventDef : Def
+{
+    public string Severity { get; init; } = EventSeverities.Info;
+
+    public bool Pause { get; init; }
+
+    /// <summary>Client toast lifetime in milliseconds. Zero means it stays until dismiss.</summary>
+    public int TtlMs { get; init; }
+
+    public string Trigger { get; init; } = "";
+
+    public string Action { get; init; } = EventActions.None;
+}