Summary

• preserve the matched paragraph structure on the first plain paragraph of block Markdown replacements
• continue resetting later paragraphs and explicitly structured Markdown
• add heading and list regression coverage

Fixes #838.

Testing

• go test ./internal/cmd -run 'TestDocsFindReplace_Markdown(BlockPreservesFirstPlainParagraphStyle|ResetsInheritedParagraphStyle|CodeBlockStartsFreshParagraphWhenInline)$' -count=1
• make ci
• autoreview: clean
• live Google Docs proof: original heading style and list ID preserved on the first replacement paragraph; subsequent paragraphs reset; scratch document trashed