Tag for “Custom” axis should come from Glyphs

@schriftgestalt, could Glyphs have a user-editable property for the four-letter tag of custom design axes? Currently, glyphsLib/fontmake uses hardcoded `XXXX` for the “Custom” axis. If Glyphs users could enter a proper four-letter axis tag, glyphsLib can copy this tag into its output designspaces and we’ll have one less quirk in the font production pipeline.


Start by tracing how glyphsLib/fontmake reads custom axes and writes designspaces, then check whether the Glyphs source format exposes a user-provided four-letter tag. Done means a valid tag from Glyphs is copied into the output designspace instead of hardcoded XXXX; the issue names no files or tests.

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