chore: mark .observe extension targets as @deprecated

Add @deprecated and @Private JSDoc annotations to all 4 POS observe
extension targets:
- pos.transaction-complete.event.observe
- pos.cash-tracking-session-start.event.observe
- pos.cash-tracking-session-complete.event.observe
- pos.cart-update.event.observe

Each annotation includes the deprecation version (2026-04), migration
path to pos.app.ready.data + shopify.addEventListener(), and a link
to the new target's documentation. pos.cart-update.event.observe also
references shopify.cart.current.subscribe() as an alternative for
render targets.

Resolves: shop/issues-retail#26066
Epic: shop/issues-retail#26019

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: vctrchu

packages/ui-extensions/src/surfaces/point-of-sale/extension-targets.ts

@@ -29,15 +29,48 @@ import type {BasicComponents} from './components/targets/BasicComponents';
 import type {TransactionCompleteWithReprintData} from './event/data';
 
 export interface EventExtensionTargets {
+  /**
+   * Fires when a transaction completes successfully.
+   *
+   * @deprecated Deprecated as of `2026-04`. Use `pos.app.ready.data` target with
+   * `shopify.addEventListener('transaction_complete', callback)` instead.
+   * See https://shopify.dev/docs/api/pos-ui-extensions/unstable/targets/pos-app-ready-data
+   * @private
+   */
   'pos.transaction-complete.event.observe': (
     data: TransactionCompleteData,
   ) => Promise<BaseOutput>;
+  /**
+   * Fires when a cash tracking session starts.
+   *
+   * @deprecated Deprecated as of `2026-04`. Use `pos.app.ready.data` target with
+   * `shopify.addEventListener('cash_tracking_session_start', callback)` instead.
+   * See https://shopify.dev/docs/api/pos-ui-extensions/unstable/targets/pos-app-ready-data
+   * @private
+   */
   'pos.cash-tracking-session-start.event.observe': (
     data: CashTrackingSessionStartData,
   ) => Promise<BaseOutput>;
+  /**
+   * Fires when a cash tracking session completes.
+   *
+   * @deprecated Deprecated as of `2026-04`. Use `pos.app.ready.data` target with
+   * `shopify.addEventListener('cash_tracking_session_complete', callback)` instead.
+   * See https://shopify.dev/docs/api/pos-ui-extensions/unstable/targets/pos-app-ready-data
+   * @private
+   */
   'pos.cash-tracking-session-complete.event.observe': (
     data: CashTrackingSessionCompleteData,
   ) => Promise<BaseOutput>;
+  /**
+   * Fires when the cart is updated.
+   *
+   * @deprecated Deprecated as of `2026-04`. Use `shopify.cart.current.subscribe()` instead,
+   * available on any render target. For background observation, use `pos.app.ready.data`
+   * target with `shopify.addEventListener('cart_update', callback)`.
+   * See https://shopify.dev/docs/api/pos-ui-extensions/unstable/targets/pos-app-ready-data
+   * @private
+   */
   'pos.cart-update.event.observe': (
     data: CartUpdateEventData,
   ) => Promise<BaseOutput>;