fix: guard serializer-shaped raw route objects

[matcldr]

/claim #30

Summary

• catch raw object route returns before response serialization inspects framework response markers
• preserve the existing ctx.json({...}) guidance for JSON-shaped objects that contain fields like serializeToResponse
• keep valid native Response and ctx.json(...)/WinterSpecResponse returns unchanged

Validation

• npx ava tests/errors/do-not-allow-raw-json.test.ts --verbose
• npx prettier --check src/create-with-winter-spec.ts src/middleware/with-response-object-check.ts tests/errors/do-not-allow-raw-json.test.ts
• npm run typecheck
• git diff --check