feat: add optional student nickname for informal display

Keep legal first and last names for formal lists; show DisplayFirstName on teams, calendars, and import matching so two Josiahs can be told apart.

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

Tests/Parsers/AssignmentRequirement_Tests.cs

@@ -1,3 +1,9 @@
+using System.Text;
+using Core.Entities;
+using Core.Models;
+using Core.Parsers;
+using Tests.Builders;
+
 namespace Tests.Parsers;
 
 public class AssignmentRequirement_Tests
@@ -16,4 +22,53 @@ public class AssignmentRequirement_Tests
 
 		Assert.Pass();
 	}
+
+	[Test]
+	public void Parse_MatchesNicknameColumnsForTwoJosiahs()
+	{
+		StudentBuilder.ResetIdCounter();
+		var jo = StudentBuilder.Create("Josiah", "Brown").WithNickname("Jo").Build();
+		var josiahB = StudentBuilder.Create("Josiah", "Green").WithNickname("Josiah B").Build();
+		var coding = EventDefinitionBuilder.Individual("Coding").WithShortName("Coding").Build();
+
+		const string csv = """
+			,Jo,josiah b
+			Coding,i,x
+			""";
+
+		var requirements = ParseCsv(csv, [coding], [jo, josiahB]);
+
+		Assert.That(requirements, Has.Length.EqualTo(2));
+		Assert.That(requirements.Single(r => r.Student.Id == jo.Id).Requirement, Is.EqualTo(Requirement.Include));
+		Assert.That(requirements.Single(r => r.Student.Id == josiahB.Id).Requirement, Is.EqualTo(Requirement.Exclude));
+	}
+
+	[Test]
+	public void Parse_SharedLegalFirstName_TakesFirstStudent()
+	{
+		StudentBuilder.ResetIdCounter();
+		var first = StudentBuilder.Create("Josiah", "Brown").WithNickname("Jo").Build();
+		var second = StudentBuilder.Create("Josiah", "Green").WithNickname("Josiah B").Build();
+		var coding = EventDefinitionBuilder.Individual("Coding").WithShortName("Coding").Build();
+
+		const string csv = """
+			,Josiah
+			Coding,i
+			""";
+
+		var requirements = ParseCsv(csv, [coding], [first, second]);
+
+		Assert.That(requirements, Has.Length.EqualTo(1));
+		Assert.That(requirements[0].Student.Id, Is.EqualTo(first.Id));
+	}
+
+	private static AssignmentRequirement[] ParseCsv(
+		string csv,
+		ICollection<EventDefinition> events,
+		ICollection<Student> students)
+	{
+		using var reader = new StreamReader(new MemoryStream(Encoding.UTF8.GetBytes(csv)));
+		using var parser = new AssignmentRequirementParser(reader);
+		return parser.Parse(events, students);
+	}
 }