fix(GRO-1171): restore UAT_PLAYBOOK and add coatType/petSizeCategory to buildPet

Address QA review findings on PR #12:
- Add coatType and petSizeCategory to buildPet defaults in packages/db/src/factories.ts
  to fix TypeScript typecheck failure
- Restore UAT_PLAYBOOK.md (was deleted during monorepo extraction) and add
  §4.15 Buffer Rules test cases

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/db/src/factories.ts

@@ -103,6 +103,8 @@ export function buildPet(overrides: Partial<PetRow> & { clientId: string }): Pet
     photoKey: null,
     photoUploadedAt: null,
     image: null,
+    coatType: null,
+    petSizeCategory: null,
     createdAt: new Date("2025-01-01T00:00:00Z"),
     updatedAt: new Date("2025-01-01T00:00:00Z"),
   };