Add binary exporters and template manager

- Native PDF exporter (cover, TOC, bookmarks, images, code, tables,
  text blocks); validated under Ghostscript
- Animated GIF exporter (title card, title overlay, progress bar)
- Image bundle exporter with watermark compositing
- DOCX and PPTX emitters (hand-built OOXML over our zip writer) with
  structural + relationship + XML well-formedness validation in tests
- Per-format template manager with .sfglt share archives
- Unified export dispatcher covering all nine formats
- 10 more workflow tests (52 total)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/util.js

@@ -98,7 +98,8 @@ function slugify(text, fallback = 'untitled') {
   const slug = String(text || '')
     .normalize('NFKD')
     .replace(/[̀-ͯ]/g, '')
-    .replace(/[^a-zA-Z0-9._ -]+/g, '')
+    .toLowerCase()
+    .replace(/[^a-z0-9._ -]+/g, '')
     .trim()
     .replace(/\s+/g, '-')
     .slice(0, 80);