Google Sans CI broken after googlesans#729

[anthrotype]

The "Build GS" CI job is broken since googlefonts/googlesans#729 (import-arabic-poc) merged earlier today (2026-05-18). The new Arabic FEA files use two constructs that fea-rs rejects but feaLib accepts:

1. <anchor NULL> without mark @class on a ligature component
2. valueRecordDef inside a feature block instead of at the top-level

Both have been accepted by feaLib since the beginning.
I have two PRs ready that fix each.

[anthrotype]

without mark @Class on a ligature component

fea-rs required mark @class after every anchor, but NULL anchors mean "no attachment here" and don't take a mark class. The FEA spec grammar requires mark @class after every anchor, but it is inconsistent with its own GPOS type 5 example wihch uses a final <anchor NULL>; without it. feaLib has followed this since its original implementation (Dec 2015).

valueRecordDef inside a feature block instead of at the top-level

The spec says this is a top-level-only statement, but feaLib has always accepted this in feature and lookup blocks for 11 years, and Google Sans Arabic is a real font that relies on this. It's trivial to support so we should.

[anthrotype]

"Build GS" CI job is green again https://github.com/googlefonts/fontc/actions/runs/26046192588/job/76570913022