docs: atlases are fixed-cell grids; state the constraint instead of discovering it

docs/superpowers/specs/2026-09-28-plot-interaction-and-fonts-design.md

@@ -17,6 +17,7 @@ Five changes to the shipped library, driven by using it:
 ## Non-goals
 
 - Arbitrary font features: no kerning, no shaping, no hinting, no subpixel positioning. Explicitly deferred.
+- **Proportional (variable-width) text.** An atlas is a fixed-cell grid: every glyph occupies one cell of the same size. That is what keeps the staged instance at 6 floats and the vertex shader free of a per-glyph size, and it is why a proportional font must draw its glyphs into a uniform cell instead of trimming the atlas rect. The constraint is structural — declared by the backend, asserted in debug builds — not a note, because violating it silently draws every glyph at the first staged glyph's size.
 - Text selection/editing, data export, annotations beyond what exists.
 - A general constraint/layout engine — `Figure` is a fixed vertical stack, not a docking system.