[dblock]

Adds the requested optional second string to MarkdownSection.replace rather than a separate substitution method:

await section.replace("Entire section body.", { cwd });
await section.replace("Old wording", "New wording", { cwd });
await section.replace(/version (\S+)/g, "release $1", { cwd });

The existing whole-body form is unchanged. The search/replacement form uses JavaScript string/regex replacement semantics only within the selected body, including nested subsections. It preserves surrounding content, LF/CRLF style and unmatched spacing; unmatched searches do not rewrite the file. Invalid inputs, ambiguous headings and malformed line endings fail before writing.

Adds shared source/installed behavior assertions, positive/negative declaration contracts, documentation, and a changelog entry. Validation: 88 tests via npm run test:coverage, npm run lint, git diff --check, and strict-engine consumers installed from the exact retained archive. The Markdown module has 100% line, branch and function coverage.

Follow-up to #14, which merged while implementation was in progress. This layer targets #16 and extends existing native stack #17 without rewriting merged history. No publication or merge performed.

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