Add print or export workflow for one-slide-per-page output

[lreading]

Summary

Post-v1 backlog item: add an export path for slide decks, starting with browser print/PDF output where each slide maps cleanly to one page.

This should stay grounded in open web output before considering office document formats.

Alternatives considered

Defer PowerPoint/LibreOffice-style formats unless there is a strong user need and a maintainable implementation path.

Affected packages

• app (presentation renderer)
• cli (@slide-spec/cli)
• docs (documentation site)
• shared (types and validation)
• Unsure