Cell

One rendered terminal cell.

symbol is a String, not a Char, because a cell can hold a multi-codepoint grapheme cluster (combining characters, emoji ZWJ sequences). A wide (two-column) grapheme lives in its left cell; the cell to its right is a continuation filler that backends skip when flushing.

That filler is an ordinary Cell.Empty — a Cell carries no continuation state of its own. It is Buffer that records which of its cells are fillers, so the same value means "blank" in one position and "the right half of the grapheme next door" in another.