add rio-grid readme

Author: raphamorim

rio-grid/README.md

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+# rio-grid
+
+Rio's shared grid-emission layer: turns a row of terminal cells plus its
+resolved styles into the GPU cell primitives (`CellBg` / `CellText`)
+that [Rio](https://github.com/raphamorim/rio)'s renderer draws. One
+pass emits backgrounds, one run-building pass emits glyphs (shaping
+runs, drawable sprites, underlines), with selection, search-highlight,
+and hint-label handling applied per row.
+
+It sits between the terminal core
+([`rio-vt`](https://crates.io/crates/rio-vt)) and the renderer, and is
+consumed by both the Rio terminal itself and `libsugarloaf`'s C ABI so
+embedders share the exact same emission logic.