Function diff

Compute the minimal set of cell-level updates needed to transform prev into next.

• If prev is null or its dimensions differ from next's, every visible cell of next is emitted (a full repaint).
• Otherwise, only cells whose grapheme, fg, bg, width, or attribute bitmask changed are emitted.
• Continuation slots (width 0) are never emitted directly — they are re-painted as a side-effect of writing the leading wide grapheme.