Move README heading names into map.json, warn about unmapped README sections, document the README-explorer workflow

site/tools/coverage.mjs

@@ -0,0 +1,20 @@
+// Warns (never fails) about README sections that have no place on the map yet.
+// Usage: node site/tools/coverage.mjs        Sections listed in map.json "offMap" (titles or group names) are skipped.
+import { readFileSync } from 'node:fs';
+import { parseRoadmap } from '../parser.js';
+import { linkDiagram } from '../diagram.js';
+
+const read = (path) => readFileSync(new URL(path, import.meta.url), 'utf8');
+const map = JSON.parse(read('../map.json'));
+const data = parseRoadmap(read('../../README.md'));
+const plan = linkDiagram(map, data);
+
+const drawn = new Set(plan.boxes.flatMap((b) => b.topics));
+const onMap = (t) => drawn.has(t) || t.children.some(onMap);
+const skipped = new Set(map.offMap || []);
+const missing = data.root.children.filter((t) => !skipped.has(t.title) && !skipped.has(t.group) && !onMap(t));
+
+for (const t of missing) {
+  console.log(`::warning file=README.md,title=Not on the roadmap map::"${t.title}" has no box on the map. Add it to site/map.json, or to "offMap" if it should stay off the map (see CONTRIBUTING.md).`);
+}
+console.log(missing.length ? `${missing.length} README section(s) are not on the map.` : 'Every README section is on the map or listed in "offMap".');