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/image-bundle.js

@@ -0,0 +1,54 @@
+'use strict';
+
+const fs = require('node:fs');
+const path = require('node:path');
+const { guideSlug, writeStepImages } = require('./common');
+const raster = require('../core/raster');
+const { decodePng, encodePng } = require('../core/png');
+
+/**
+ * Image bundle exporter: one annotated PNG per image step plus a
+ * metadata.json describing the guide, with an optional watermark overlay.
+ */
+
+const DEFAULT_TEMPLATE = {
+  watermarkPath: '', // PNG overlaid bottom-right when set
+  watermarkOpacity: 0.6,
+};
+
+function exportImageBundle(ast, outDir, template = {}) {
+  const tpl = { ...DEFAULT_TEMPLATE, ...template };
+  fs.mkdirSync(outDir, { recursive: true });
+  const images = writeStepImages(ast, outDir);
+
+  if (tpl.watermarkPath && fs.existsSync(tpl.watermarkPath)) {
+    const mark = decodePng(fs.readFileSync(tpl.watermarkPath));
+    const alpha = Math.round(255 * Math.max(0, Math.min(1, tpl.watermarkOpacity)));
+    for (const { relPath } of images.values()) {
+      const file = path.join(outDir, relPath);
+      const img = decodePng(fs.readFileSync(file));
+      const faded = raster.cloneImage(mark);
+      for (let i = 3; i < faded.data.length; i += 4) {
+        faded.data[i] = Math.round((faded.data[i] * alpha) / 255);
+      }
+      raster.drawImage(img, faded, Math.max(0, img.width - mark.width - 12), Math.max(0, img.height - mark.height - 12));
+      fs.writeFileSync(file, encodePng(img));
+    }
+  }
+
+  const meta = {
+    format: 'stepforge-image-bundle',
+    version: 1,
+    guide: { title: ast.guide.title, generatedAt: ast.generatedAt },
+    steps: ast.steps.map((step) => ({
+      number: step.number,
+      title: step.title,
+      image: images.get(step.stepId)?.relPath || null,
+    })),
+  };
+  const metaFile = path.join(outDir, `${guideSlug(ast)}-bundle.json`);
+  fs.writeFileSync(metaFile, JSON.stringify(meta, null, 2) + '\n');
+  return { file: metaFile, imageCount: images.size };
+}
+
+module.exports = { exportImageBundle, DEFAULT_TEMPLATE };