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/pyproject.toml

@@ -7,6 +7,7 @@ requires-python = ">=3.10"
 dependencies = [
     "django>=5.0",
     "django-ninja>=1.0",
+    "django-readers>=2.0",
     "pydantic>=2.0",
     "PyJWT>=2.0",
 ]