Cross-check el() ids against index.html in the selftest

A knob wired in app.js but missing from the markup throws during wiring, aborts
the module and leaves a blank page - a symptom pointing nowhere near its cause.
It has now happened twice, so it gets a check rather than vigilance: the
selftest fetches both files and compares the id sets in each direction.

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

js/selftest.js

@@ -45,6 +45,37 @@ const spreadX = (frames, slot) => {
 
 /* ---- the tests ---- */
 
+// Cross-check every el('id') in app.js against the ids in index.html.
+//
+// This bug class has bitten twice: a knob wired in app.js but absent from the
+// markup throws during wiring, which aborts the rest of the module and leaves a
+// blank page. The symptom ("nothing happens") points nowhere near the cause, so
+// it is worth an automated check rather than vigilance.
+export async function runWiring() {
+  const out = [];
+  try {
+    const [app, html] = await Promise.all([
+      fetch('./js/app.js').then((r) => r.text()),
+      fetch('./index.html').then((r) => r.text()),
+    ]);
+    const ids = new Set([...app.matchAll(/\bel\(\s*['"]([\w-]+)['"]\s*\)/g)].map((m) => m[1]));
+    const list = app.match(/for \(const id of \[([\s\S]*?)\]\)/);
+    if (list) {
+      for (const m of list[1].matchAll(/'([\w]+)'/g)) { ids.add(m[1]); ids.add(m[1] + 'v'); }
+    }
+    const have = new Set([...html.matchAll(/id="([\w-]+)"/g)].map((m) => m[1]));
+    const missing = [...ids].filter((i) => !have.has(i));
+    out.push({ name: `every el() id exists in index.html (${ids.size} checked)`,
+               pass: missing.length === 0, detail: missing.join(', ') });
+    const unused = [...have].filter((i) => !ids.has(i));
+    out.push({ name: 'no orphaned ids in index.html', pass: unused.length === 0,
+               detail: unused.join(', ') });
+  } catch (e) {
+    out.push({ name: 'wiring check ran', pass: false, detail: e.message });
+  }
+  return out;
+}
+
 export function run() {
   results.length = 0;