fix: a page is the paper's exact height, not the pixel it rounds to

The sheet boxes rounded cm × PX_PER_CM where --user-page-height keeps it
exact, and pageBreaks builds its grid from the rounded value they publish
as --pb-section-page: A4 came out 1123px against 1122.52, so every page
ran 0.127mm long and the text crept down the flow — 1.8mm by the
sixteenth page, 6.0mm by the forty-eighth, against a reference whose
footer sits at the same height on every page.
Corpus 1772 -> 1108, eight fixtures improved, none rose.

src/lib/components/CLAUDE.md

@@ -110,6 +110,9 @@ a 177-page document. Word and LibreOffice draw only the visible pages; we can't.
 - `min-width: 0` on the row and its scroller: a flex item's automatic minimum is its
   content, so without it the grid widens the whole app instead of scrolling.
 - No ruler here: it would repeat per column and eat into the page's own box.
+- **`sectionPaper` stays unrounded.** `pageBreaks` builds its grid from the height this
+  publishes as `--pb-section-page`, so rounding A4's 1122.52px to 1123 ran every page
+  0.127mm long — 6mm of drift by page 48, against a reference page that never moves.
 
 ## Zoom (`Editor.svelte`)
 

src/lib/components/Editor.svelte

@@ -197,11 +197,12 @@ import { EMPTY_PAGE_DECOR, type PageDecor } from '../storage/pageDecor';
   ].map((g) => g.map((n) => Math.round(n)).join('|')).join(','));
 
   // Each section's own paper (px). A section that names neither format nor orientation
-  // is on the document's, so its entry is the document's box.
+  // is on the document's, so its entry is the document's box. Unrounded: this is the
+  // grid pageBreaks places against, and half a pixel per page accumulates down the document.
   let sectionPaper = $derived([
     pageDimsCm(pageFormat, orientation),
     ...extraHfSections.map((s) => pageDimsCm(s.format ?? pageFormat, s.orientation ?? orientation)),
-  ].map((d) => ({ w: Math.round(d.w * PX_PER_CM), h: Math.round(d.h * PX_PER_CM) })));
+  ].map((d) => ({ w: d.w * PX_PER_CM, h: d.h * PX_PER_CM })));
   // The sheet .paper reserves is the widest of them: a landscape section is wider than
   // the portrait ones around it and would otherwise be cut at the page edge.
   let paperWidth = $derived(Math.max(...sectionPaper.map((p) => p.w)));