Markdown writer (with `smart`) turns a lone em/en dash into a thematic break

When the Markdown writer has `smart` enabled (the default for `-t markdown`), an em dash is written as `---` and an en dash as `--`. If those dashes end up alone on a line, e.g. a paragraph whose only content is an em dash, the emitted `---` re-parses as a thematic break (`HorizontalRule`). This is a **semantic** round-trip change (`Para` becomes `HorizontalRule`), not just a cosmetic one. ## pandoc version ``` pandoc 3.9.0.2 ``` ## Steps to reproduce A paragraph whose sole content is an em dash (`—`, U+2014): ```console $ printf '—\n' | pandoc -f markdown -t native [ Para [ Str "\8212" ] ] $ printf '—\n' | pandoc -f markdown -t markdown --- $ printf '—\n' | pandoc -f markdown -t markdown | pandoc -f markdown -t native [ HorizontalRule ] ``` Pure AST round-trip (`native → markdown → native`) shows the same loss: ```console $ printf '[Para [Str "—"]]\n' | pandoc -f native -t markdown | pandoc -f markdown -t native [ HorizontalRule ] ``` ## Expected Round-tripping `Para [Str "—"]` through the Markdown writer and reader should yield `Para [Str "—"]` again (as it does for any other character). The block structure must not change. ## Actual The paragraph becomes a thematic break (`HorizontalRule`).