SIXEL measured canvas dimensions

Trap: passing a measured dashboard canvas to the generic image encoder looks sufficient because it accepts maxWidth. Fix: use its explicit preserve-dimensions/allowWide path only after dashboard MAX_CANVAS_PIXELS validation, or return complete ASCII. The normal encoder safety caps must remain for arbitrary attachments, but silently downscaling a valid dashboard width or height breaks its cell-derived grid geometry; Sixel geometry must use actual positive terminal columns, never the ASCII 80-column minimum.