Kanbun: school-standard return marks, enforced by the checker

[HanpuLi]

Return marks in all 29 readings were checked against the school-standard rules:

• a return over one character uses レ;
• a 一二 pair inside another 一二 pair makes the outer pair 上下;
• 上下 is used only around a complete 一二 pair.

Four places used a valid reading order in a non-standard form. They are corrected, and the checker now rejects all three patterns.

Text	Before	After
甲五 preface	記下…始無上レ避焉 (上下 with no inner 一二)	記二…始無一レ避焉
甲九 preface	得下某所二寄一舊稿上	得二某所レ寄舊稿一
甲九 preface	與三篋中所二存一	與二篋中所一レ存
乙三 body	恐三花為レ我辭二根一	恐二花為レ我辭一レ根

The reading order is unchanged in every case, so the 書き下し text is untouched. Only three frames change: a5, a9 and b3.

There are no 甲乙 or 天地人 marks anywhere. The lines are short and never nest past 上下.

• tools/poetry_kunten.py check_marks checks bodies and titles. It rejects adjacent 二一 and adjacent 下上, 一二 nested in 一二, and 上下 without an inner 一二. Before the fix, it rejects the old data on 甲五.
• The pinned b3 order in poetry_kunten_manuscript.py is updated to the standard form.
• docs/poetry-kunten-collation.md records the change. The full-line alignment doc is regenerated.

Full npm run qa passes.

🤖 Generated with Claude Code

https://claude.ai/code/session_014ECg6ksCbqKTUJNKX4Qt6C