v2.5.7 — NPDE distribution plot + PNG export

New GOF features

• NPDE distribution plot — a new "NPDE Dist" tab appears in the GOF panel whenever the loaded table file contains an NPDE column. Shows a histogram with a normal density overlay and n / mean / SD statistics, styled identically to the CWRES Hist panel. When NPDE is absent the button is hidden automatically; it reappears if you load a table that contains it
• PNG export on all matplotlib GOF panels — CWRES Hist, QQ Plot and NPDE Dist each now have a "Save PNG…" button (300 DPI via matplotlib savefig); the button is disabled until a plot has been rendered