CVE-2026-91189 GUIX binary resource loader: unvalidated glyph range causes an integer underflow and a heap out-of-bounds write when loading an untrusted font or theme

The GUIX binary resource loader never checks that a font page's glyph range is well formed. gxpageheaderfirstglyph and gxpageheaderlastglyph are read verbatim from the file and only the upper bound of lastglyph is validated, so a page declaring firstglyph lastglyph makes last - first + 1 underflo...