[glyphs] Generate STAT axis values from instances, matching glyphsLib >= 6.14

glyphsLib now auto-generates STAT data from a Glyphs source's instances (googlefonts/glyphsLib#1158, in v6.14.0): one AxisValue per distinct instance coordinate on each axis, the default value elided, and a STAT-only `ital` axis pinned at 0, or 1 for an italic family.
It also honours the `Export STAT Table`, `Elidable STAT Axis Value Name` and `Style Name as STAT entry` custom parameters.

fontc writes only a stub with design axis records but no axis values (see `fontbe/src/stat.rs` comment "we support only a very simple STAT at time of writing"), unless a STAT table block arrives via FEA.

So as soon as we bump past the pinned glyphsLib 6.13.1, every Glyphs source with Variable Font Settings instances will get a STAT that differs from fontmake's.

Start in fontbe/src/stat.rs, especially the comment that only a simple STAT table is supported, and compare the requested behavior with glyphsLib 6.14. Review how Glyphs instances and custom parameters reach fontc, including FEA-provided STAT data. Done means generated STAT tables include the described axis values and ital behavior while honoring the listed parameters.

Written by the indexing model from the issue text.