hadn't spotted that 'ingenious and innovate' positioning of the only guaranteed source of high-frequency noise, the crystal, w.r.t. the ADC inputs. A useful observation.
Not the ADC inputs, the AVCC power supply pin. This feature seems to be maintained across all the packages, actually. Maybe it makes sense; the crystal oscillator WOULD be considered analog electronics; perhaps it and some of the other more obviously analog circuitry legitimately share power supplies, and perhaps any oscillator-related noise is at frequencies far above anything relevant to the ADC anyway. But... It's not making me feel warm and fuzzy!