Fixes a silent-corruption bug found by rendering samples and looking at them.
Font coverage was probed on the logical string while the renderer draws presentation forms on non-Raqm builds. Fonts like Fustat and Mada cover base Arabic via GSUB and carry no presentation-form glyphs, so the probe reported 100% coverage and every glyph rendered as an empty box — with the label still claiming the text.
Also fixed: the coverage probe skipped table cells and list items entirely, and the fallback path handed documents a font that could not draw their script at all.
466 tests. Non-Raqm builds only; Raqm builds were never affected.