Curve-curve intersection in corner components doesn't work

This matches fonttools, but if we encountered a source containing this particular case (which is constructable, if perhaps not super useful) we could crash (as would fonttools)

See https://github.com/googlefonts/glyphsLib/blob/f90e4060bafbb62ea76d269e77ef2823cb57190a/Lib/glyphsLib/filters/cornerComponents.py#L142-L148,

> In practice curved corner segments meeting curved glyph segments is uncommon, but it is supported in the glyphs sources.

_Originally posted by @cmyr in https://github.com/googlefonts/fontc/pull/1721#discussion_r2491110500_

Start by comparing fontc's corner-component handling with glyphsLib's cornerComponents.py at lines 142-148 and the discussion in fontc PR 1721. Reproduce the constructable source case described in the issue; done means curved corner segments meeting curved glyph segments no longer cause a crash.

Written by the indexing model from the issue text.