Add Sprache package and enhance event occurrence parsing with new grammar rules

Core/Models/EventOccurrenceParseResult.cs

@@ -51,6 +51,11 @@ public class ParsingIssue
     /// </summary>
     public int LineNumber { get; set; }
 
+    /// <summary>
+    /// The column number where the issue occurred (1-based, 0 if not available).
+    /// </summary>
+    public int ColumnNumber { get; set; }
+
     /// <summary>
     /// The actual line content where the issue occurred.
     /// </summary>
@@ -65,6 +70,16 @@ public class ParsingIssue
     /// Human-readable description of the issue.
     /// </summary>
     public string Message { get; set; } = string.Empty;
+
+    /// <summary>
+    /// What was expected at the error location (e.g., "month name", "MS or HS", "time value").
+    /// </summary>
+    public string Expected { get; set; } = string.Empty;
+
+    /// <summary>
+    /// What was actually found at the error location.
+    /// </summary>
+    public string Found { get; set; } = string.Empty;
 }
 
 /// <summary>