RenderStateRowCells.Graphemes previously allocated a fresh codepoint slice for every text cell. That made plaintext renderers pay allocation and conversion costs even when most cells contained a single ASCII codepoint. Add GraphemesInto so callers can reuse their own codepoint scratch buffer, and add AppendGraphemes for renderers that want UTF-8 bytes directly. The existing Graphemes API now delegates through the reusable path while preserving its nil result for empty cells.