refactor(native): keep the Android bridge additions apart from the iOS ones in trekShell.ts

The new methods sit at the end of the interface and the restore's Android
answer has a type of its own, so the watch and large-screen branches, which
add their methods after storeRestore, merge without a conflict.

client/src/native/iap.ts

@@ -1,5 +1,5 @@
 import { isNativeAndroid, isNativeIos } from './platform';
-import { TrekShell } from './trekShell';
+import { TrekShell, type StoreRestoreResult } from './trekShell';
 
 /**
  * In-app purchase, from the app's side. A seam, like {@link TrekShell}: the
@@ -162,7 +162,10 @@ export const nativeIapClient: IapClient = {
   },
   async restore() {
     try {
-      const { jws, purchaseToken, productId, transactionId } = await TrekShell.storeRestore({ prefix: PRODUCT_PREFIX });
+      // Google Play answers with a purchase token where the App Store answers with a signed transaction.
+      const { jws, purchaseToken, productId, transactionId }: StoreRestoreResult = await TrekShell.storeRestore({
+        prefix: PRODUCT_PREFIX,
+      });
       if (purchaseToken && transactionId) {
         return { platform: 'google', productId: productId ?? '', receipt: purchaseToken, transactionId };
       }

client/src/native/trekShell.ts

@@ -96,10 +96,12 @@ export interface TrekShellPlugin {
   storeFinish(options: { transactionId: string }): Promise<void>;
   /** iOS, feature wallet: adds a .pkpass (base64) to Apple Wallet, or opens it there when it is already in. */
   addToWallet(options: { data: string }): Promise<{ result: 'added' | 'opened' | 'cancelled' }>;
-  /** The newest active subscription under `prefix` on this Apple ID or Google account, if any. */
-  storeRestore(options: {
-    prefix: string;
-  }): Promise<{ jws?: string; purchaseToken?: string; productId?: string; transactionId?: string }>;
+  /** iOS, feature storeKit: the newest active subscription under `prefix` on this Apple ID, if any. */
+  storeRestore(options: { prefix: string }): Promise<{ jws?: string; transactionId?: string }>;
+  addListener(
+    event: 'pushOpened',
+    listener: (payload: { url: string }) => void
+  ): Promise<{ remove: () => Promise<void> }>;
   /** Android, feature googleWallet: opens a Google Wallet save link (https://pay.google.com/gp/v/save/...). */
   addToGoogleWallet(options: { url: string }): Promise<{ result: 'opened' }>;
   /** Android, feature windowLayout: the window's size classes and a foldable's posture and hinge. */
@@ -108,10 +110,14 @@ export interface TrekShellPlugin {
     event: 'windowLayout',
     listener: (payload: NativeWindowLayout) => void
   ): Promise<{ remove: () => Promise<void> }>;
-  addListener(
-    event: 'pushOpened',
-    listener: (payload: { url: string }) => void
-  ): Promise<{ remove: () => Promise<void> }>;
+}
+
+/** What Google Play Billing answers a restore with (Android, feature playBilling), beside iOS's `jws`. */
+export interface StoreRestoreResult {
+  jws?: string;
+  purchaseToken?: string;
+  productId?: string;
+  transactionId?: string;
 }
 
 /** A document shared to TREK, as the native side reads it. */