PDFsharp/MigraDoc layout and font state is process-wide. Parallel exports
of the same snapshot laid text out differently (for example merged words
like "Total physicalsheets" and shifted table columns). Serialize the
whole render-and-save in NestPdfWriter.Write; reports are rare.
Regression test exports one snapshot 128 times in parallel and compares
page content streams with a sequential export; it failed with dozens of
mismatches per run before the lock.
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