Columns on selected text go in their own section (#2024) (#2071)

With text selected, Layout > Columns used to change the whole section. As in
Word (observed black-box), a continuous section break now goes where the
selection starts and one where it ends, splitting a paragraph there, unless that
end already meets a section boundary (the document's start or end, or an
existing break); the columns go on the section between, in one undo step.
`layout.columns` gains `apply: "selection"`, its default when text is selected
in the main text. More Columns... offers "Selected text" and picks it then.

Co-authored-by: Brandon Thomas <echelon@gmail.com>

docs/ui-parity.md

@@ -1,6 +1,6 @@
 # UI parity with Microsoft Word
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-18 · **Change:** trivial (Different First Page headers reachable from page 1, #2826; previously trivial: a copied equation also goes in the clipboard's MathML formats, #1297; previously trivial: Read Mode keeps the keyboard, shows whole columns and keeps text clear of the page-turn bar, #1772); previously trivial: font list entries of tall-ascent fonts sit in their row, not cut off, #2570); previously trivial: Split Cells and Convert Text to Table dialogs follow Word's merged-cell limits and Initial column width, #2592; previously trivial: a new source's Tag is made like Word's, three letters of the author and two digits of the year with a number added when taken, and an edit keeps the Tag, #2094; previously trivial: Web Layout text fills the window width, #1876; previously trivial: Language dialog keeps inherited proofing language and no-proof choices (#2717)); previously trivial: comment balloons readable in the dark theme, #2045; previously trivial (Modify Table Style formats every table style part, #714); previously trivial: a click on the ribbon chrome hands the keyboard to the page in the same frame, #1093 follow-up; previously trivial: Page Setup vertical alignment, #1320; previously trivial: the app opens on the Home tab every launch, as in Word; previously trivial: no document behind the Start screen, #1129; previously trivial: equation pictures share a few textures, #1002; previously trivial: selection bar in the left margin, #856; previously trivial: table column and row markers on the rulers, #700; previously trivial: table borders and the resize handle drag on the page, #217; previously major: first version; ribbon, dialogs, shortcuts and on-canvas interaction measured from `crates/ui-egui` and the command registry) · **Target:** Microsoft Word (Microsoft 365) for Mac 16.113.4
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-18 · **Change:** trivial (Columns on selected text, #2024; previously trivial: Different First Page headers reachable from page 1, #2826; previously trivial: a copied equation also goes in the clipboard's MathML formats, #1297; previously trivial: Read Mode keeps the keyboard, shows whole columns and keeps text clear of the page-turn bar, #1772); previously trivial: font list entries of tall-ascent fonts sit in their row, not cut off, #2570); previously trivial: Split Cells and Convert Text to Table dialogs follow Word's merged-cell limits and Initial column width, #2592; previously trivial: a new source's Tag is made like Word's, three letters of the author and two digits of the year with a number added when taken, and an edit keeps the Tag, #2094; previously trivial: Web Layout text fills the window width, #1876; previously trivial: Language dialog keeps inherited proofing language and no-proof choices (#2717)); previously trivial: comment balloons readable in the dark theme, #2045; previously trivial (Modify Table Style formats every table style part, #714); previously trivial: a click on the ribbon chrome hands the keyboard to the page in the same frame, #1093 follow-up; previously trivial: Page Setup vertical alignment, #1320; previously trivial: the app opens on the Home tab every launch, as in Word; previously trivial: no document behind the Start screen, #1129; previously trivial: equation pictures share a few textures, #1002; previously trivial: selection bar in the left margin, #856; previously trivial: table column and row markers on the rulers, #700; previously trivial: table borders and the resize handle drag on the page, #217; previously major: first version; ribbon, dialogs, shortcuts and on-canvas interaction measured from `crates/ui-egui` and the command registry) · **Target:** Microsoft Word (Microsoft 365) for Mac 16.113.4
 
 How WordCraft looks and feels next to Word: ribbon, dialogs, panes, keyboard, mouse and touch.
 Typing behaviour has its own checklist ([`typing-parity.md`](typing-parity.md)). All assets are our
@@ -71,6 +71,7 @@ The Language dialog reads effective character formatting, including document def
 | 2026-10-16 | trivial | Font list: each entry's capitals are centred in its row, so a font with a very tall ascent (Amiri, Amiri Quran) no longer sits low or is cut in half, and the list reads as one even list (#2570) |
 | 2026-10-16 | trivial | Read Mode keeps the keyboard on entering, shows only whole columns, centred, and ends them above the page-turn bar (#1772) |
 | 2026-10-16 | trivial | Comment and revision balloons are readable in the dark theme: the card, the markup area, the author stripes and the leader lines come from theme tokens, and the text reaches 4.5:1 (#2045) |
+| 2026-10-15 | trivial | Layout › Columns works on the selected text, as in Word: section breaks around it, the rest of the document unchanged; More Columns… adds "Selected text" to Apply to and picks it when text is selected (#2024) |
 | 2026-10-13 | trivial | New/Modify Table Style: No Border on a part that inherits its lines hides them; emptying the font box makes the part's font inherited again (#1288) |
 | 2026-10-14 | trivial | Tab stops edited on the ruler: corner kind selector, click to set, drag to move or off to clear, per-kind markers mirrored for RTL; each gesture one `para.tabs` call and one undo step; Rulers 85% → 90% |
 | 2026-10-12 | trivial | Tables on the page: one selected cell (Select Cell, or a click along its left edge) is a cell selection when its border is dragged: the border moves in its row only (#962) |