feat(store): load/save and target accessors with validation

tests/localstorage-shim.js

@@ -0,0 +1,9 @@
+export function createLocalStorage() {
+  const data = new Map();
+  return {
+    getItem(k) { return data.has(k) ? data.get(k) : null; },
+    setItem(k, v) { data.set(k, String(v)); },
+    removeItem(k) { data.delete(k); },
+    clear() { data.clear(); },
+  };
+}