largeop ascent/descent when using a vertical glyph assembly

From https://docs.microsoft.com/en-us/typography/opentype/spec/math:

"Note that for assemblies growing in the vertical direction, the distribution of height between ascent and descent is not defined. The math-layout engine is responsible for positioning the resulting assembly relative to the baseline."

Per https://mathml-refresh.github.io/mathml-core/#the-glyphassembly-table there are two cases:

(1) "Set B to the advanced width, ascent and descent of the glyph of id MathGlyphVariantRecord.variantGlyph. "

(2) "If there is valid GlyphAssembly subtable then set B to the glyph assembly width, glyph assembly ascent and glyph assembly descent for the target size T." which is itself "If GlyphAssembly is vertical, the width is the maximum advance width of the glyphs of id GlyphPartRecord.glyphID for all the GlyphPartRecord in GlyphAssembly.partRecords, the ascent is the glyph assembly stretch size for a given target size T and the descent is 0."

So in case of a largeop glyph assembly, the descent is always 0. I don't think people care about this case, but just opening to be sure...

(For stretchy glyph assembly the ascent/descent of the mo is later calculated from the non-stretchy siblings)

Read the OpenType MATH and MathML Core glyph assembly sections cited in the issue, comparing vertical largeop and stretchy cases. Confirm the intended ascent/descent behavior and document the agreed clarification in the specification; no repository file or test is named.