feat(pos): add storage.latest subscribable types to Storage interface

[vctrchu]

What

Add SubscribableStorage type and optional current property to the POS Storage interface for reactive cross-target communication.

Why

Extensions running across multiple targets (tile, modal, background) need to share state reactively. The storage.current subscribable enables one target to subscribe to a key and react when another target updates it via set() or delete().

TAG PR: ui-api-design#1416
Issue: shop/issues-retail#26020

How

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

• SubscribableStorage<T> — mapped type where each key maps to ReadonlySignalLike<T | undefined>, reusing the existing signal type from shared.ts (consistent with cart, connectivity, scanner, locale APIs)
• current?: SubscribableStorage<BaseStorageTypes> — optional property on Storage interface exposing each storage key as a subscribable

// Synchronous access to the current value:
const status = shopify.storage.current.syncStatus.value;

// Subscribe to changes from another target:
const unsubscribe = shopify.storage.current.syncStatus.subscribe((value) => {
  console.log('syncStatus changed:', value);
});

Also updates ui-extensions-tester mock (createStorage()) with a Proxy-based current implementation.

Companion PR

• pos-mobile runtime: shop/world#554969 — signal registry + proxy + storageApi integration

Base Branch

2026-07-rc — ships with the 2026-07 API version.

[github-actions]

🚨🚨🚨 Docs migration in progress 🚨🚨🚨

We are actively migrating UI extension reference docs to MDX in the areas/platforms/shopify-dev zone of the monorepo. This impacts docs for the following surfaces:

• Admin UI extensions
• App Home
• Checkout UI extensions
• Customer account UI extensions
• POS UI extensions

During this migration, please be aware of the following:

.doc.ts files are being deprecated. Changes to .doc.ts files in this repo will not be reflected in the new MDX-based docs. If you need to update docs for a reference that has already been migrated, make your changes directly in the areas/platforms/shopify-dev zone of the monorepo instead.

Doc comments in .ts source files (the comment blocks above types and functions) are also affected. Generating docs from these comments currently requires a newer version of the @shopify/generate-docs library that isn't yet available. Updates to doc comments may not produce the expected output until the migration is complete.

Examples that previously lived in this repo are being moved to the areas/platforms/shopify-dev zone of the monorepo and should be authored there going forward.

What should I do?

• If your PR includes changes to .doc.ts files, doc comments, or examples, please reach out to us in #devtools-proj-templated-refs so we can help ensure your updates are captured correctly.
• If your PR is limited to source code changes (non-docs), you can ignore this notice.

Thanks for your patience while we complete the migration! 🙏

[vctrchu]

/snapit