Restructure src/ and tests/ from package-by-type to package-by-domain

All classes are now organised by domain (Availability, Booking, Auth).
Each domain package contains its value object, repository, admin controller,
REST endpoint, and any shortcode pages under a matching sub-namespace.
Cross-cutting wiring (Plugin, AdminMenu, RestRegistrar, ShortcodeRegistrar,
Schema) lives at src/ root. Tests mirror the domain structure.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/Unit/Booking/LessonTest.php

@@ -0,0 +1,68 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Booking;
+
+use Unsupervised\Schedular\Booking\Lesson;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class LessonTest extends TestCase
+{
+    public function testStatusConstants(): void
+    {
+        self::assertSame('pending', Lesson::STATUS_PENDING);
+        self::assertSame('confirmed', Lesson::STATUS_CONFIRMED);
+        self::assertSame('cancelled', Lesson::STATUS_CANCELLED);
+    }
+
+    public function testValidStatusesContainsAllThree(): void
+    {
+        self::assertContains(Lesson::STATUS_PENDING, Lesson::VALID_STATUSES);
+        self::assertContains(Lesson::STATUS_CONFIRMED, Lesson::VALID_STATUSES);
+        self::assertContains(Lesson::STATUS_CANCELLED, Lesson::VALID_STATUSES);
+        self::assertCount(3, Lesson::VALID_STATUSES);
+    }
+
+    public function testConstructorDefaults(): void
+    {
+        $lesson = new Lesson(slotId: 1, studentId: 2, instructorId: 3);
+
+        self::assertSame(Lesson::STATUS_PENDING, $lesson->status);
+        self::assertNull($lesson->notes);
+        self::assertNull($lesson->id);
+    }
+
+    public function testFromRowMapsCorrectly(): void
+    {
+        $row = (object) [
+            'id'            => '99',
+            'slot_id'       => '10',
+            'student_id'    => '20',
+            'instructor_id' => '30',
+            'status'        => 'confirmed',
+            'notes'         => 'Bring your guitar.',
+        ];
+
+        $lesson = Lesson::fromRow($row);
+
+        self::assertSame(99, $lesson->id);
+        self::assertSame(10, $lesson->slotId);
+        self::assertSame(20, $lesson->studentId);
+        self::assertSame(30, $lesson->instructorId);
+        self::assertSame('confirmed', $lesson->status);
+        self::assertSame('Bring your guitar.', $lesson->notes);
+    }
+
+    public function testToArrayContainsExpectedKeys(): void
+    {
+        $lesson = new Lesson(1, 2, 3, Lesson::STATUS_PENDING, 'Note', 5);
+        $arr    = $lesson->toArray();
+
+        self::assertArrayHasKey('id', $arr);
+        self::assertArrayHasKey('slot_id', $arr);
+        self::assertArrayHasKey('student_id', $arr);
+        self::assertArrayHasKey('instructor_id', $arr);
+        self::assertArrayHasKey('status', $arr);
+        self::assertArrayHasKey('notes', $arr);
+    }
+}