feat: browser-native PDF export via Print with animations completed

[sonihemish]

Summary

Makes the browser's built-in Print → Save as PDF (Cmd/Ctrl+P) produce a correct one-page-per-slide PDF with all entrance animations completed — no scripts, no Playwright, no dependencies. Addresses #28.

Generated decks also get a small floating 📥 PDF button wired to window.print(), so viewers can export without knowing the shortcut.

Why

The existing export-pdf.sh works, but it requires Node + Playwright (~150MB Chromium download), produces raster screenshots (no selectable text, large files), and can't be used by someone who just received the HTML file. The repo already had a @media print section in viewport-base.css, but in practice it was broken:

1. No @page rule → slides printed on A4/Letter with margins, scaled and cut.
2. Entrance animations never completed → .reveal elements on slides the presenter hadn't visited stayed at opacity: 0, so printed pages were missing text and diagrams.
3. No print-color-adjust → browsers stripped themed backgrounds.
4. Blank page after every slide → any slide box even 1px taller than the page sheet (e.g. padding without border-box) makes browsers emit a trailing blank page per slide.

What changed

• viewport-base.css — @page sized to the 1920×1080 design canvas; print-color-adjust: exact; fast-forward .reveal variants and [data-anim] to their finished state; freeze transitions/animations mid-flight; hide deck/editing chrome; harden the slide box (border-box + max-height + zero margin) against the blank-page problem.
• html-template.md — setupPrintExport() in the SlidePresentation controller: beforeprint marks every slide .visible so class-gated animations resolve, afterprint restores the exact pre-print state. Documents the default floating PDF button.
• bold-template-pack/deck-stage.js — same treatment for the web-component path: the head-injected print stylesheet (slide content is light DOM that ::slotted() can't reach) gains the animation fast-forward and hardening; beforeprint/afterprint handlers cover styles gated on [data-deck-active].
• SKILL.md / README.md — browser-native export documented as the primary zero-dependency path; the Playwright script kept as the automation alternative.

On-screen rendering is untouched — every change is scoped to @media print/@page, and the JS handlers snapshot and fully restore deck state after the print dialog closes. Verified with byte-identical before/after screen screenshots.

Testing

Headless Chromium --print-to-pdf (same engine as the Cmd+P dialog) against a template-faithful test deck using .reveal, .reveal-scale, .reveal-left, and .reveal-blur on slides never visited on screen:

• ✅ Exactly N pages for N slides, each 1920×1080, no blank pages
• ✅ All animation-gated text and SVG diagrams present on unvisited slides
• ✅ Deck chrome excluded; themed backgrounds preserved
• ✅ Pathological case (content-box slide with padding, previously → blank page after every slide) fixed
• ✅ Screen rendering byte-identical before/after (SHA-256 of full-page screenshots)
• ✅ Also validated against 14 real-world generated decks (4–9 slides each): every PDF page count matched its slide count with zero empty pages

Notes

• Complements rather than replaces export-pdf.sh — the script remains for automation and non-Chromium browsers.
• Vector output means selectable/searchable text and much smaller files than the screenshot pipeline.

Closes #28.