Add a heading-levels rule to paper organization

Ezra's rule (2026-10-30): a page's title is its level 1 heading, so the body starts at level 2, and each heading is at most one level deeper than the one before it.
Pages split out of larger pages kept the headings of the page they came from, which skipped levels, started pages too deep, or repeated the title.

The rule has no home in psw yet; the gha check that enforces it is Morrison-Lab/gha#1373.