Add shapes tests and fix django-readers as core dependency

25 tests covering Shape, Diff, and NestedDiff:
- Shape metaclass: model resolution, field extraction, nested detection, spec/pair building
- Query: list, filter, nested relations, empty results, Pydantic serialization
- Diff (new): detects all fields as changed
- Diff (existing): no changes, single field, multiple fields, nonexistent ID
- Diff (nested): created, updated, deleted, combined, nonexistent relation

Fixes:
- django-readers moved from optional to core dependency
- Shape import lazy-loaded via __getattr__ (django_readers imports
  contenttypes which can't happen during apps.populate())
- Added Author, Book, Tag test models

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

django/tests/models.py

@@ -38,3 +38,37 @@ class EmailUser(AbstractBaseUser, PermissionsMixin):
 
     class Meta:
         app_label = "tests"
+
+
+# ─── Shape test models ──────────────────────────────────────────────────────
+
+
+class Author(models.Model):
+    name = models.CharField(max_length=100)
+    bio = models.TextField(blank=True, default="")
+
+    class Meta:
+        app_label = "tests"
+
+    def __str__(self):
+        return self.name
+
+
+class Book(models.Model):
+    title = models.CharField(max_length=200)
+    pages = models.IntegerField(default=0)
+    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name="books")
+
+    class Meta:
+        app_label = "tests"
+
+    def __str__(self):
+        return self.title
+
+
+class Tag(models.Model):
+    name = models.CharField(max_length=50)
+    books = models.ManyToManyField(Book, related_name="tags", blank=True)
+
+    class Meta:
+        app_label = "tests"