docs: sync annotation parity through v31

[Osilvfe]

Absorb the previously prepared v31 documentation sync now that the implementation is on main.

• update replication status from thirty to thirty-one parity passes
• document shared parent word-mask ownership across ruby, base glyphs and per-word romanization
• document ruby-segment timing / UTF-16 sweep semantics and annotation-aware mask geometry
• remove the stale remaining-work note that deeper ruby/roman annotation-mask parity is still missing
• update the Chinese integration guide with the same annotation-mask behavior

This PR changes documentation only; it intentionally does not touch the later v32-v36 implementation/tooling work already on main.