[dblock]

Summary

• Add RubyGemRelease::Markdown::Section with exact-heading validation, full-body replace!, and scoped gsub!(pattern, replacement) for changing matching body text.
• gsub! accepts strings and regular expressions with capture references, preserves other body text and surrounding sections, normalizes inserted line endings, and does not write when nothing matches. Heading boundaries remain intact even when substitutions match boundary whitespace or insert into an empty section.
• Share Commonmarker source parsing with changelogs; reject missing/duplicate headings and ignore quoted, code, and HTML examples when locating headings.
• Replace the lifecycle example README regular expression with the helper and document both editing methods.

Validation

• bundle exec rake: 342 examples, no failures; RuboCop clean.
• bundle exec rake build.
• Built-archive API check verifies heading boundaries, scoped substitution, capture references, CRLF preservation, and an absent final newline without RSpec.

Created by an AI on behalf of @dblock.