ASCII 流程图画板

Keep a sparse map from integer grid coordinates to characters and render only the visible region onto a canvas with a monospace font. For the line tool, compute an L-shaped path between drag start and end; for boxes, the four edges. After placing characters, recompute each affected cell's glyph from which neighbours are lines, choosing the right corner or junction character from a lookup table. Support pan and zoom by transforming the viewport, and export by scanning the bounding box into text.