Add core domain layer: schema, store, sanitizer, placeholders, settings

- Folder-based GuideStore with atomic writes, trash/restore, duplicate,
  substep reparenting, folders/favorites, working-image crop/reset
- Allowlist HTML sanitizer applied on every store write
- Placeholder scopes (guide > global > system) and collection
- Persisted app settings with deep default merge
- 16 workflow tests exercising real on-disk round-trips

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

tests/unit/helpers.js

@@ -0,0 +1,22 @@
+'use strict';
+
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+
+/** Create a unique temp directory, auto-registered for cleanup by caller. */
+function makeTmpDir(label) {
+  return fs.mkdtempSync(path.join(os.tmpdir(), `stepforge-${label}-`));
+}
+
+function rmrf(dir) {
+  fs.rmSync(dir, { recursive: true, force: true });
+}
+
+// Minimal valid 1x1 red PNG, used where pixel content doesn't matter.
+const TINY_PNG = Buffer.from(
+  'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8BQDwAEhQGAhKmMIQAAAABJRU5ErkJggg==',
+  'base64'
+);
+
+module.exports = { makeTmpDir, rmrf, TINY_PNG };