feat(pos): add keys property to Storage type for subscribable access

Adds Subscribable, StorageKeys types and optional keys property to the
POS Storage interface. Each key is exposed as Subscribable<T | undefined>
for reactive updates across targets of the same extension.

Part of: shop/issues-retail#26020

Author: vctrchu

packages/ui-extensions/src/surfaces/point-of-sale/types/storage.ts

@@ -13,6 +13,25 @@ export class StorageError extends Error {
 export interface Storage<
   BaseStorageTypes extends Record<string, any> = Record<string, unknown>,
 > {
+  /**
+   * Reactive access to storage values as Subscribables.
+   *
+   * Each key is exposed as a `Subscribable<T | undefined>`, enabling reactive
+   * updates across targets of the same extension. One target can subscribe to
+   * a key and react when another target updates it via `set()` or `delete()`.
+   *
+   * Only available on API version `2026-04` and later.
+   *
+   * @example
+   * ```typescript
+   * // Subscribe to changes from another target:
+   * const unsubscribe = shopify.storage.keys.syncStatus.subscribe((value) => {
+   *   console.log('syncStatus changed:', value);
+   * });
+   * ```
+   */
+  keys?: StorageKeys<BaseStorageTypes>;
+
   /**
    * Stores a value under the specified key, overwriting any existing value. Values must be JSON-serializable and return `StorageError` when storage limits are exceeded. Commonly used for storing user preferences, caching API responses, or passing contextual data from tiles to modals.
    *
@@ -73,3 +92,31 @@ export interface Storage<
     Keys extends keyof StorageTypes = keyof StorageTypes,
   >(): Promise<[Keys, StorageTypes[Keys]][]>;
 }
+
+/**
+ * Represents a readonly value managed by the host that an extension can
+ * subscribe to for reactive updates.
+ */
+export interface Subscribable<Value = unknown> {
+  /** Synchronous access to the current value. */
+  readonly value: Value;
+  /** Registers a callback that fires when the value changes. Returns an unsubscribe function. */
+  subscribe: (callback: (value: Value) => void) => () => void;
+}
+
+/**
+ * Provides reactive, subscribable access to individual storage keys.
+ *
+ * Each property is a `Subscribable` that reflects the current value of the
+ * corresponding storage key. Values are `undefined` when the key does not exist.
+ *
+ * Mutations are performed through the existing `Storage` methods (`set`, `delete`,
+ * `clear`, etc.) — `StorageKeys` is read-only and reactive.
+ */
+export type StorageKeys<
+  BaseStorageTypes extends Record<string, any> = Record<string, unknown>,
+> = {
+  readonly [K in keyof BaseStorageTypes]: Subscribable<
+    BaseStorageTypes[K] | undefined
+  >;
+};