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>

core/renderast.js

@@ -0,0 +1,123 @@
+'use strict';
+
+const fs = require('node:fs');
+const path = require('node:path');
+const { sanitizeHtml } = require('./sanitize');
+const { htmlToText, deepClone } = require('./util');
+const { systemPlaceholders, resolveScopes, expandPlaceholders } = require('./placeholders');
+const { decodePng } = require('./png');
+const { renderAnnotations, applyFocusedView } = require('./raster');
+
+/**
+ * The Render AST is the single normalized document model every exporter
+ * consumes. It resolves placeholders, hierarchical numbering, hidden/skipped
+ * filtering, and absolute image paths — exporters never read the store.
+ */
+
+function buildRenderAst(store, guideId, { globals = {}, now = new Date(), maxSteps = 0 } = {}) {
+  const guide = store.getGuide(guideId);
+  const stepsMap = store.listSteps(guideId);
+
+  const includedIds = guide.stepsOrder.filter((id) => {
+    const s = stepsMap.get(id);
+    if (!s || s.hidden) return false;
+    if (s.skipped && guide.flags.hideSkippedStepsInExports) return false;
+    return true;
+  });
+
+  const values = resolveScopes({
+    guide,
+    globals,
+    system: systemPlaceholders(guide, { now, stepCount: includedIds.length }),
+  });
+  const expand = (text) => expandPlaceholders(text, values);
+
+  const steps = [];
+  const topCounter = { n: 0 };
+  const childCounters = new Map();
+  const numberOf = new Map();
+
+  for (const id of includedIds) {
+    const raw = stepsMap.get(id);
+    const step = deepClone(raw);
+    let number;
+    let depth = 0;
+    if (step.parentStepId && numberOf.has(step.parentStepId)) {
+      const parentNo = numberOf.get(step.parentStepId);
+      const c = (childCounters.get(step.parentStepId) || 0) + 1;
+      childCounters.set(step.parentStepId, c);
+      number = `${parentNo}.${c}`;
+      depth = number.split('.').length - 1;
+    } else {
+      step.parentStepId = null; // orphan substeps render top-level
+      topCounter.n += 1;
+      number = String(topCounter.n);
+    }
+    numberOf.set(step.stepId, number);
+
+    const ast = {
+      stepId: step.stepId,
+      parentStepId: step.parentStepId,
+      number,
+      depth,
+      kind: step.kind,
+      status: step.status,
+      skipped: step.skipped,
+      forceNewPage: Boolean(step.forceNewPage),
+      title: expand(step.title || ''),
+      descriptionHtml: sanitizeHtml(expand(step.descriptionHtml || '')),
+      descriptionText: htmlToText(expand(step.descriptionHtml || '')),
+      focusedView: step.focusedView,
+      annotations: (step.annotations || []).map((a) => ({ ...a, text: expand(a.text || '') })),
+      textBlocks: (step.textBlocks || []).map((tb) => ({
+        ...tb,
+        title: expand(tb.title || ''),
+        descriptionHtml: sanitizeHtml(expand(tb.descriptionHtml || '')),
+        descriptionText: htmlToText(expand(tb.descriptionHtml || '')),
+      })),
+      codeBlocks: step.codeBlocks || [],
+      tableBlocks: step.tableBlocks || [],
+      links: step.links || [],
+      image: null,
+    };
+    if (step.image) {
+      const absPath = path.join(store.stepDir(guideId, step.stepId), step.image.workingPath);
+      if (fs.existsSync(absPath)) {
+        ast.image = { absPath, width: step.image.size.width, height: step.image.size.height };
+      }
+    }
+    steps.push(ast);
+  }
+
+  const limited = maxSteps > 0 ? steps.slice(0, maxSteps) : steps;
+
+  return {
+    format: 'stepforge-render-ast',
+    version: 1,
+    generatedAt: now.toISOString(),
+    placeholders: values,
+    guide: {
+      id: guide.guideId,
+      title: expand(guide.title),
+      descriptionHtml: sanitizeHtml(expand(guide.descriptionHtml || '')),
+      descriptionText: htmlToText(expand(guide.descriptionHtml || '')),
+      createdAt: guide.createdAt,
+      updatedAt: guide.updatedAt,
+      flags: guide.flags,
+    },
+    steps: limited,
+  };
+}
+
+/**
+ * Decode a step's working image and burn in annotations + focused view.
+ * Returns an RGBA raster image, or null for steps without images.
+ */
+function renderStepImage(astStep) {
+  if (!astStep.image) return null;
+  const base = decodePng(fs.readFileSync(astStep.image.absPath));
+  const annotated = renderAnnotations(base, astStep.annotations);
+  return applyFocusedView(annotated, astStep.focusedView);
+}
+
+module.exports = { buildRenderAst, renderStepImage };