Handle CYGNSS mask opt‑out and bump ASCAT max obs limit

[amfox37]

1. Comment out static QC mask in CYGNSS reader
2. Guard CYGNSS reader clean‑up so mask arrays are only deallocated when allocated.
3. Raise ASCAT max_obs limit from 250k to 280k to accommodate larger number of observations on the very rare occasions (only once in 24 years?) when 3 observation files fall within the assimilation window.