Interface TextQuad

One glyph's rectangle: where it goes, and what it samples.

Positions are world units in the layout's own frame (baseline-left origin, §7a Y-up); texture coordinates are the atlas cell, normalized 0…1. Corners are given as a minimum/maximum pair rather than as four vertices because every consumer — a quad builder, a sprite, an SVG rect — wants the rectangle, and expanding it to vertices is one line at the point of use.