Prose lines shaped like tree guides ("├── item") wrapped by restarting
at column 0, shearing the tree apart — doubly fast for CJK text. Wrap
the node text within the cells remaining after the guide prefix and
re-emit the prefix on continuation rows in pass-through form (├ → │,
└ → blank), so ancestor rails stay joined. Paragraph/text blocks only;
fitting lines, non-tree prose, and code blocks are byte-for-byte
unchanged.
