Keep percentage flex columns on the same HTML row

[HarryCordewener]

Summary

• reserve inter-item spacing before resolving percentage flex bases in HTML, matching terminal layout
• include divider border and padding within each flex item’s declared size
• add a regression test for two 50% columns separated by │

This fixes the HTML-only wrap exposed while preparing the screenshots in #42.

Verification

• dotnet build MarkupString.slnx --no-restore --disable-build-servers
• dotnet run --no-build --project MarkupString.Tests (1,031 passed)

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📥 Commits

Reviewing files that changed from the base of the PR and between 52bd924 and a4f3e36.

📒 Files selected for processing (3)

• MarkupString.Html/Emitters/LayoutHtml.cs
• MarkupString.Html/LayoutCss.cs
• MarkupString.Tests/Layout/BlockLayoutTests.cs

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