Add dependency-free browser test suite and README

49 tests run in headless Chrome via tests/run.sh: unit tests for the pure
modules plus integration tests that drive the real page in an iframe
(filtering, URL state, bag, limits, cross-tab sync, checkout, dialogs,
accessibility and overflow checks).

tests/harness.js

@@ -0,0 +1,52 @@
+/** A minimal test harness: no dependencies, runs in any browser. */
+
+const tests = [];
+
+export const test = (name, fn) => tests.push({ name, fn });
+
+export function assert(condition, message = "assertion failed") {
+  if (!condition) throw new Error(message);
+}
+
+export function equal(actual, expected, message = "") {
+  const a = JSON.stringify(actual);
+  const e = JSON.stringify(expected);
+  if (a !== e) throw new Error(`${message ? `${message}: ` : ""}expected ${e}, got ${a}`);
+}
+
+/** Assert that `fn` throws (sync or async); optionally that its message matches `pattern`. */
+export async function throws(fn, pattern) {
+  try {
+    await fn();
+  } catch (error) {
+    if (pattern && !pattern.test(String(error && error.message))) {
+      throw new Error(`threw "${error.message}", expected to match ${pattern}`);
+    }
+    return;
+  }
+  throw new Error("expected a throw, but nothing was thrown");
+}
+
+export const sleep = (ms) => new Promise((resolve) => setTimeout(resolve, ms));
+
+export async function runAll() {
+  const log = document.getElementById("log");
+  let failed = 0;
+  for (const { name, fn } of tests) {
+    const item = document.createElement("li");
+    try {
+      await fn();
+      item.className = "pass";
+      item.textContent = name;
+    } catch (error) {
+      failed++;
+      item.className = "fail";
+      item.textContent = `${name} -- ${error && error.message}`;
+      console.error(name, error);
+    }
+    log.append(item);
+  }
+  const summary = document.getElementById("summary");
+  summary.textContent = `${failed ? "FAIL" : "PASS"} ${tests.length - failed}/${tests.length} tests`;
+  document.documentElement.dataset.result = failed ? "fail" : "pass";
+}