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/Entities/Student_Tests.cs

@@ -0,0 +1,57 @@
+using Tests.Builders;
+
+namespace Tests.Entities;
+
+[TestFixture]
+public class Student_Tests
+{
+    [Test]
+    public void DisplayFirstName_FallsBackToFirstName()
+    {
+        var student = StudentBuilder.Create("Josiah", "Brown").Build();
+
+        Assert.That(student.DisplayFirstName, Is.EqualTo("Josiah"));
+        Assert.That(student.ToString(), Is.EqualTo("Josiah"));
+    }
+
+    [Test]
+    public void DisplayFirstName_UsesTrimmedNickname()
+    {
+        var student = StudentBuilder.Create("Josiah", "Brown").WithNickname("  Jo  ").Build();
+
+        Assert.That(student.DisplayFirstName, Is.EqualTo("Jo"));
+        Assert.That(student.ToString(), Is.EqualTo("Jo"));
+    }
+
+    [Test]
+    public void LegalNameProperties_IgnoreNickname()
+    {
+        var student = StudentBuilder.Create("Josiah", "Brown").WithNickname("Jo").Build();
+
+        Assert.That(student.FirstName, Is.EqualTo("Josiah"));
+        Assert.That(student.LastName, Is.EqualTo("Brown"));
+        Assert.That(student.Name, Is.EqualTo("Josiah Brown"));
+        Assert.That(student.FirstNameLastName, Is.EqualTo("Josiah Brown"));
+        Assert.That(student.LastNameFirstName, Is.EqualTo("Brown, Josiah"));
+    }
+
+    [Test]
+    public void DisplayFirstName_TreatsWhitespaceAsUnset()
+    {
+        var student = StudentBuilder.Create("Josiah", "Brown").WithNickname("   ").Build();
+
+        Assert.That(student.DisplayFirstName, Is.EqualTo("Josiah"));
+    }
+
+    [Test]
+    public void NormalizeNickname_ClearsBlankAndTrims()
+    {
+        var blank = StudentBuilder.Create("Josiah", "Brown").WithNickname("  ").Build();
+        blank.NormalizeNickname();
+        Assert.That(blank.Nickname, Is.Null);
+
+        var nick = StudentBuilder.Create("Josiah", "Brown").WithNickname("  Jo  ").Build();
+        nick.NormalizeNickname();
+        Assert.That(nick.Nickname, Is.EqualTo("Jo"));
+    }
+}