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.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# Run the browser test suite headlessly and exit non-zero on any failure.
+# Needs python3 (static server) and Google Chrome; override with CHROME=/path/to/chrome.
+# To see the results in a browser instead: python3 -m http.server, then open /tests/
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+CHROME="${CHROME:-/Applications/Google Chrome.app/Contents/MacOS/Google Chrome}"
+PORT="${PORT:-8765}"
+PROFILE="$(mktemp -d)"
+
+python3 -m http.server "$PORT" --bind 127.0.0.1 >/dev/null 2>&1 &
+SERVER=$!
+trap 'kill "$SERVER" 2>/dev/null || true; rm -rf "$PROFILE"' EXIT
+sleep 1
+
+DOM="$("$CHROME" --headless=new --no-sandbox --disable-gpu --user-data-dir="$PROFILE" \
+  --virtual-time-budget=30000 --dump-dom "http://127.0.0.1:$PORT/tests/" 2>/dev/null)"
+
+printf '%s\n' "$DOM" | grep -o '<li class="[a-z]*">[^<]*' \
+  | sed -e 's/<li class="pass">/  ok    /' -e 's/<li class="fail">/  FAIL  /'
+printf '%s\n' "$DOM" | grep -o 'id="summary">[^<]*' | sed 's/.*>//'
+printf '%s\n' "$DOM" | grep -q 'data-result="pass"'