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>

exporters/index.js

@@ -0,0 +1,31 @@
+'use strict';
+
+const { exportJson } = require('./json');
+const { exportMarkdown } = require('./markdown');
+const { exportHtmlSimple, exportHtmlRich } = require('./html');
+const { exportPdf } = require('./pdf');
+const { exportGifGuide } = require('./gif');
+const { exportImageBundle } = require('./image-bundle');
+const { exportDocx } = require('./docx');
+const { exportPptx } = require('./pptx');
+
+/** Unified dispatch: format id -> exporter(ast, outDir, templateOptions). */
+const EXPORTERS = {
+  json: exportJson,
+  markdown: exportMarkdown,
+  'html-simple': exportHtmlSimple,
+  'html-rich': exportHtmlRich,
+  pdf: exportPdf,
+  gif: exportGifGuide,
+  'image-bundle': exportImageBundle,
+  docx: exportDocx,
+  pptx: exportPptx,
+};
+
+function runExport(format, ast, outDir, templateOptions = {}) {
+  const exporter = EXPORTERS[format];
+  if (!exporter) throw new Error(`unknown export format: ${format}`);
+  return exporter(ast, outDir, templateOptions);
+}
+
+module.exports = { EXPORTERS, runExport };