- flow.rs: an opening bracket starting a wrapped line is set flush unless the paragraph is
right to left, as before the split (it had become !vertical, so vertical text with
LineEndHalf lost the flush and RTL paragraphs gained it). PlacementContext carries rtl.
- flow.rs, ruby.rs: put back the comments the split dropped (JLREQ, burasagari, Optical
Margin Alignment, kashidas, ruby phases).
- xtask root(): the parent directory, not .../xtask/..