DocumentBuilder renders ~~strike~~ as bold + strike (affects PDF and DOCX export) #1373

Split out of #1309's "Related, found during the same work and NOT fixed", so it is not lost when that issue closes for the contents-page-number regression.

DocumentBuilder.ReadInlinesInto sets isStrong from DelimiterCount >= 2, which is also true for ~~strike~~ — so strikethrough text is emitted as bold + strike rather than strike alone.

The delimiter character has to be part of the test: **/__ is strong, ~~ is strikethrough, and only the former should set isStrong.

Affects both renderers, since both consume the same document model:

• PDF — DocumentPrintComposer.RenderInline wraps the run in <strong><s>…</s></strong>
• DOCX — DocxDocumentRenderer writes both run properties

Pre-existing; faithfully reproduced by #1230 rather than silently corrected.

Where: src/MeshWeaver.Markdown.Export/Ast/DocumentBuilder.cs
Test home: test/MeshWeaver.Markdown.Export.Test/DocumentBuilderTests.cs