Add Render AST and text exporters: JSON, Markdown, HTML simple/rich

- Render AST: placeholder expansion, hierarchical numbering (1, 1.1),
  hidden/skipped filtering, preview step limit, annotated image rendering
- JSON exporter with sidecar annotated PNGs
- Markdown exporter: TOC with resolving anchors, text blocks as
  blockquotes, fenced code, tables, Azure-wiki image sizing option
- Self-contained HTML exporters (data-URI images, zero external refs);
  rich variant adds floating TOC, checkboxes, localStorage progress
- HTML->Markdown converter for the sanitizer-allowed tag set
- 7 exporter workflow tests (42 total)

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

exporters/common.js

@@ -0,0 +1,55 @@
+'use strict';
+
+const fs = require('node:fs');
+const path = require('node:path');
+const { slugify } = require('../core/util');
+const { encodePng } = require('../core/png');
+const { renderStepImage } = require('../core/renderast');
+
+/**
+ * Shared exporter helpers: every image-bearing exporter renders annotated
+ * step images through the same pipeline so output is consistent.
+ */
+
+function guideSlug(ast) {
+  return slugify(ast.guide.title, 'guide');
+}
+
+function imagesDirName(ast) {
+  return `steps-${guideSlug(ast)}`;
+}
+
+/**
+ * Render every image step to an annotated PNG inside outDir/<steps-slug>/.
+ * Returns Map stepId -> { relPath, width, height }.
+ */
+function writeStepImages(ast, outDir) {
+  const dirName = imagesDirName(ast);
+  const dir = path.join(outDir, dirName);
+  const result = new Map();
+  let n = 0;
+  for (const step of ast.steps) {
+    n += 1;
+    const img = renderStepImage(step);
+    if (!img) continue;
+    fs.mkdirSync(dir, { recursive: true });
+    const fileName = `${String(n).padStart(3, '0')}-${slugify(step.title || step.stepId, step.stepId)}.png`;
+    fs.writeFileSync(path.join(dir, fileName), encodePng(img));
+    result.set(step.stepId, { relPath: `${dirName}/${fileName}`, width: img.width, height: img.height });
+  }
+  return result;
+}
+
+/** Render step images in-memory (for self-contained HTML, PDF, GIF...). */
+function renderAllImages(ast) {
+  const result = new Map();
+  for (const step of ast.steps) {
+    const img = renderStepImage(step);
+    if (img) result.set(step.stepId, img);
+  }
+  return result;
+}
+
+const LEVEL_LABEL = { info: 'Note', warn: 'Warning', error: 'Important', success: 'Tip' };
+
+module.exports = { guideSlug, imagesDirName, writeStepImages, renderAllImages, LEVEL_LABEL };