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/run.js

@@ -0,0 +1,8 @@
+import { runAll } from "./harness.js";
+import "./dom.test.js";
+import "./format.test.js";
+import "./catalog.test.js";
+import "./cart.test.js";
+import "./app.test.js";
+
+await runAll();