Keep a paragraph's space after clear of the footnote separator (#1116)

[dkhlapov]

#1116: keep a paragraph's space after clear of the footnote separator

Fixes #1116. On a page with footnotes, a paragraph that ends there now keeps its space after above the footnote separator. When the space after doesn't fit, the last line goes to the next page with its note (with widow control, one more line goes too), as in Word.

• Contextual spacing and automatic spacing between list items drop the requirement.
• Pages without notes are unchanged: space after may still run past the bottom margin.
• The rule also applies to the document's last paragraph. That case hasn't been checked against Word.

Tests:

• space_after_stays_clear_of_the_footnote_separator sweeps a paragraph with 6 pt space after, and a footnote on its last line, down the page 1 pt at a time. It fails on main.
• space_after_may_overflow_the_bottom_margin_without_notes checks that pages without notes are unchanged.
• cargo xtask ci passes.

🤖 Generated with Claude Code

[dkhlapov]

Review follow-ups (7710fc7):

• Same tests as the spacing code: space_after_kept now uses the same contextual-spacing and "next paragraph gets a label" tests the spacing code uses.
• Computed only when needed: it's worked out only for a paragraph's last line on a page with notes.
• Tests: space_after_stays_clear_when_only_the_next_paragraph_has_contextual_spacing and space_after_stays_clear_before_a_numbered_paragraph_without_a_label.

Full CI passed.

[echelon]

Thanks for the contribution, @dkhlapov! This has landed in main. If you haven't already, come join us on Discord: https://discord.gg/artcraft