Is the MacBook Air's built-in calibration good enough for casual photo editing?

Regarding the MacBook Air: Yes, the Display Calibrator Assistant in macOS gets you close enough for most work without spending money. Use Expert Mode to set gamma to 2.2, white point to D65 or Native, and match brightness to your environment. The result won’t match a hardware colorimeter, but for web use, social media, and casual print proofing, it’s perfectly usable.