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>

tests/unit/fixture-guide.js

@@ -0,0 +1,53 @@
+'use strict';
+
+const { GuideStore } = require('../../core/store');
+const raster = require('../../core/raster');
+const { encodePng } = require('../../core/png');
+
+/**
+ * Build a realistic guide used by exporter tests: real PNG screenshots,
+ * annotations, substeps, text/code/table blocks, placeholders, and a
+ * hidden + skipped step to exercise filtering.
+ */
+function buildFixtureGuide(rootDir) {
+  const store = new GuideStore(rootDir);
+  const guide = store.createGuide({
+    title: 'Configure [[Product]] backups',
+    descriptionHtml: '<p>Maintained by <strong>[[Author]]</strong>.</p>',
+    placeholders: { Product: 'AcmeSync', Author: 'Casey' },
+  });
+
+  // screenshot 1: blue window with a light panel
+  const shot1 = raster.createImage(320, 200, [40, 60, 200, 255]);
+  raster.fillRect(shot1, 40, 30, 240, 140, [240, 240, 245, 255]);
+  const s1 = store.addStep(guide.guideId, {
+    title: 'Open [[Product]] settings',
+    descriptionHtml: '<p>Click the <b>gear</b> icon, then choose <a href="https://docs.example.com">Settings</a>.</p>',
+    annotations: [
+      { type: 'rect', x: 0.125, y: 0.15, w: 0.75, h: 0.7, style: { stroke: '#FF0000', strokeWidth: 6, fill: 'transparent' } },
+      { type: 'number', value: 1, x: 0.02, y: 0.05, w: 0.12, h: 0.2, style: { stroke: '#E5484D' } },
+    ],
+  }, encodePng(shot1), { width: 320, height: 200 });
+
+  const sub = store.addStep(guide.guideId, {
+    kind: 'empty',
+    parentStepId: s1.stepId,
+    title: 'Verify the gear icon is visible',
+    textBlocks: [{ position: 'after-description', level: 'warn', title: 'Access', descriptionHtml: '<p>Admins only.</p>' }],
+  });
+
+  const shot2 = raster.createImage(320, 200, [20, 140, 90, 255]);
+  const s2 = store.addStep(guide.guideId, {
+    title: 'Enable nightly backups',
+    descriptionHtml: '<p>Use the schedule below.</p>',
+    codeBlocks: [{ id: 'cb1', language: 'cron', code: '0 2 * * * /usr/local/bin/acmesync --backup' }],
+    tableBlocks: [{ id: 'tb1', rows: [['Day', 'Window'], ['Weekdays', '02:00-03:00'], ['Weekends', '04:00-05:00']] }],
+  }, encodePng(shot2), { width: 320, height: 200 });
+
+  const hidden = store.addStep(guide.guideId, { kind: 'empty', title: 'Internal-only note', hidden: true });
+  const skipped = store.addStep(guide.guideId, { kind: 'empty', title: 'Legacy path', skipped: true });
+
+  return { store, guide: store.getGuide(guide.guideId), s1, sub, s2, hidden, skipped };
+}
+
+module.exports = { buildFixtureGuide };