fix: the parity reference keeps the blank pages LibreOffice inserts

Its PDF export drops auto-inserted blank pages unless IsSkipEmptyPages=false is
passed, so a document whose chapters open on a right page was measured against a
reference several sheets short of what LibreOffice lays out: 456 pages against 468
on the largest fixture, 28 against 32 on another. The corpus total rises on the
honest reference — that is drift the old one hid, not new drift.

tests/render-parity/README.md

@@ -19,6 +19,11 @@ Exit code 1 when any document differs. `--json` also dumps both sides' lines
 | | reference | editor |
 |---|---|---|
 | render | `soffice --convert-to pdf` | Playwright Chromium, real app, real file input |
+
+The PDF is exported with `IsSkipEmptyPages=false`: LibreOffice drops its own
+auto-inserted blank pages by default — a chapter forced onto a right page — and the
+reference then has fewer sheets than the document LibreOffice lays out (the
+458-page guide 456 against 468, one fixture 28 against 32).
 | read | `pdftotext -bbox-layout` (word boxes, pt) | `Range.getClientRects()` (word boxes, px) |
 
 Both sides are normalized to mm from the top-left of each page, grouped into lines,

tests/render-parity/run.mjs

@@ -24,7 +24,11 @@ const POS_TOL_MM = 1.0;        // reported as a position difference beyond this
 function loRender(file, work) {
   execFileSync('soffice', [
     '--headless', '--norestore', `-env:UserInstallation=file://${work}/loprofile`,
-    '--convert-to', 'pdf', '--outdir', work, file,
+    // Keep the blank pages LibreOffice inserts itself (a chapter forced onto a right
+    // page): its PDF export drops them by default and the reference then loses a sheet
+    // the document really has.
+    '--convert-to', 'pdf:writer_pdf_Export:{"IsSkipEmptyPages":{"type":"boolean","value":"false"}}',
+    '--outdir', work, file,
   ], { stdio: 'pipe', timeout: 120_000 });
   const pdf = join(work, basename(file, extname(file)) + '.pdf');
   if (!existsSync(pdf)) throw new Error(`LibreOffice produced no PDF for ${file}`);