Add widescreen Markdown slide decks

[0xtlt]

Summary

• add an opt-in --slides mode that renders fixed 16:9 PDF pages;
• treat isolated Markdown --- lines as slide boundaries while preserving every other horizontal rule and existing document-mode rendering;
• add modern (default), minimal, and dark built-in templates through --slide-template;
• use full-frame covers, large left-aligned titles, editorial content pages, and current / total page counts;
• render level-three headings as tracked eyebrow labels and expand slide tables to the available width;
• retain presentation-sized code, image constraints, overflow warnings, and intentional empty slides;
• render Mermaid canvases transparently, enlarge simple diagrams in slides, and use a contrasting diagram palette on dark templates;
• add a copy-paste README example for a technical deck with a Mermaid pipeline and highlighted Rust code, plus a generated 16:9 PNG preview;
• document the workflow and template choices in English throughout the project documentation.

Compatibility

The existing A4 / Letter document path remains the default and keeps its original appearance. --slide-template requires --slides, while slide-only page options conflict with --page-size, --landscape, and --page-break-before so the fixed layout and separator semantics stay unambiguous.

Validation

• cargo fmt --check
• CARGO_BUILD_JOBS=1 CARGO_PROFILE_DEV_DEBUG=0 cargo clippy --all-targets --locked -- -D warnings
• CARGO_BUILD_JOBS=1 CARGO_PROFILE_DEV_DEBUG=0 cargo test --locked — 55 unit tests and 25 CLI integration tests passed
• CARGO_BUILD_JOBS=1 CARGO_PROFILE_DEV_DEBUG=0 RUSTDOCFLAGS='-D warnings' cargo doc --no-deps --locked
• generated four-slide PDFs for all three templates and verified every MediaBox is 960 × 540 pt (16:9)
• visually inspected covers, headings, lists, full-width tables, callouts, Mermaid diagrams, code blocks, and every page of the existing example.md A4 output
• verified overflowing covers retain their background, physical page totals remain accurate, and standalone images fit margins from 8 through 45 mm
• verified slide callouts render without a left accent rule while document callouts keep their existing appearance
• verified the built-in templates use independent warm-cream, white, and neutral-charcoal palettes without a lateral accent bar
• verified transparent, presentation-sized Mermaid diagrams on modern, minimal, and dark slide templates
• verified tall Mermaid pipelines stay on their Markdown slide and transparent architecture containers remain inside crop bounds

Demo

Download the compressed MP4 editorial-template demo (temporary link, valid for 7 days; generated exclusively from public sample content with the default #C94C35 accent).