fix: preserve whitespace-only code spans in Markdown export

[vycdev2]

Summary

• Preserve whitespace-only inline code spans with three or more spaces when exporting a Todo file to Markdown.
• Add a regression that renders the exported Markdown and checks the spaces survive.

Verification

• npm test — passed (428 tests); the new test failed on untouched develop before the fix.
• npm run compile — passed.
• npm run typecheck — passed.
• npm run format:check — passed.
• git diff --check — passed.

Risk

• Low: changes only Markdown rendering for whitespace-only inline code spans.

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