fix: preserve line endings in new archive files

[vycdev2]

Summary

• Preserve the source Todo document line ending when creating an empty archive.
• Keep existing archive content authoritative when it already establishes LF or CRLF.
• Add a regression test and changelog entry for CRLF archive creation.

Verification

• npm run compile — passed: webpack development build completed.
• npm test — passed: 230 tests.
• npm run format:check — passed.
• git diff --check — passed.

Risk

• Low: the fallback changes only empty archive content; existing archive files continue to retain their detected line endings.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.