fix: emit mirrorMargins in sectPr for Word for Mac PDF export (#74)

[forest6511]

Summary

• Defensively duplicate <w:mirrorMargins/> inside sectPr so Word for Mac's "Save as PDF" / "Print to PDF" pipeline mirrors verso pages correctly. The element is still emitted in settings.xml per OOXML spec; Word for Windows and LibreOffice ignore unknown sectPr children, so behaviour is unchanged for them.
• Unblocks KDP paperback workflows that rely on MirrorMargins + MarginGutter to satisfy the 22.225 mm inner-margin requirement (701–828 page band).

Fixes #74.

Why this works

OOXML spec (ECMA-376 §17.15.1.43) places mirrorMargins under CT_Settings, which the current implementation already does. Empirically, Word for Mac's PDF export pipeline only honours the flag when it also appears under sectPr. Word's reader is tolerant of unknown sectPr children (per the spec's Markup Compatibility rules), so duplicating is a low-risk additive change rather than an alternative-location move.

Changes

• csharp-version/src/MarkdownToDocx.Core/OpenXml/OpenXmlDocumentBuilder.cs: append <w:mirrorMargins/> to sectPr when MirrorMargins == true.
• csharp-version/tests/MarkdownToDocx.Tests/Integration/MarkdownToDocxIntegrationTests.cs: regression tests verifying both presence (when enabled) and absence (when disabled) in sectPr and settings.xml.

Note: on deserialization the OpenXml SDK exposes the in-sectPr element as OpenXmlUnknownElement (since it is not in the CT_SectPr schema), so the assertions match by LocalName + namespace URI rather than the typed MirrorMargins class.

Test plan

• dotnet test (320/320 passing locally)
• New: Build_WithMirrorMargins_ShouldEmitMirrorMarginsInBothSettingsAndSectPr
• New: Build_WithoutMirrorMargins_ShouldNotEmitMirrorMarginsAnywhere
• Manual verification on Word for Mac: open generated DOCX, File → Save as PDF, check verso page margins meet KDP 22.225 mm requirement.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.17%. Comparing base (b2672c8) to head (57ce521).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   89.16%   89.17%   +0.01%     
==========================================
  Files          28       28              
  Lines        1606     1608       +2     
  Branches      205      206       +1     
==========================================
+ Hits         1432     1434       +2     
  Misses        128      128              
  Partials       46       46              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.