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/json.js

@@ -0,0 +1,56 @@
+'use strict';
+
+const fs = require('node:fs');
+const path = require('node:path');
+const { guideSlug, writeStepImages } = require('./common');
+
+/**
+ * JSON exporter: structured guide + steps, annotated screenshots written to
+ * a sidecar steps-<title>/ folder, image paths relative to the JSON file.
+ */
+
+const DEFAULT_TEMPLATE = {
+  pretty: true,
+  includeImages: true,
+  includeAnnotations: true,
+};
+
+function exportJson(ast, outDir, template = {}) {
+  const tpl = { ...DEFAULT_TEMPLATE, ...template };
+  fs.mkdirSync(outDir, { recursive: true });
+  const images = tpl.includeImages ? writeStepImages(ast, outDir) : new Map();
+
+  const doc = {
+    format: 'stepforge-guide',
+    version: 1,
+    generatedAt: ast.generatedAt,
+    guide: {
+      title: ast.guide.title,
+      descriptionHtml: ast.guide.descriptionHtml,
+      createdAt: ast.guide.createdAt,
+      updatedAt: ast.guide.updatedAt,
+    },
+    steps: ast.steps.map((step) => ({
+      number: step.number,
+      kind: step.kind,
+      status: step.status,
+      title: step.title,
+      descriptionHtml: step.descriptionHtml,
+      descriptionText: step.descriptionText,
+      image: images.has(step.stepId) ? images.get(step.stepId) : null,
+      annotations: tpl.includeAnnotations ? step.annotations : undefined,
+      textBlocks: step.textBlocks.map((tb) => ({
+        position: tb.position, level: tb.level, title: tb.title, descriptionHtml: tb.descriptionHtml,
+      })),
+      codeBlocks: step.codeBlocks,
+      tableBlocks: step.tableBlocks,
+      links: step.links,
+    })),
+  };
+
+  const file = path.join(outDir, `${guideSlug(ast)}.json`);
+  fs.writeFileSync(file, JSON.stringify(doc, null, tpl.pretty ? 2 : 0) + '\n');
+  return { file, imageCount: images.size };
+}
+
+module.exports = { exportJson, DEFAULT_TEMPLATE };