fix: skip upgraded transactions during subscription upgrade (#20)

## Problem

When upgrading from a monthly subscription to a yearly subscription
(within the same subscription group), `onPurchaseSuccess` was emitting
old purchase objects first, and the correct upgraded transaction arrived
only after 3-4 minutes in iOS Sandbox.

- Related hyochan/react-native-iap#3054

### Root Cause

StoreKit 2's `Transaction.updates` emits **upgraded transactions** (with
`isUpgraded = true`) during subscription upgrades, in addition to the
new subscription transaction. These old transactions should be filtered
out.

## Solution

Added filtering logic to skip both revoked and upgraded transactions in
`startTransactionListener()`:

```swift
// Skip revoked or upgraded transactions (happens during subscription upgrades)
if transaction.revocationDate != nil || transaction.isUpgraded {
    OpenIapLog.debug("⏭️ Skipping revoked/upgraded transaction: \(transactionId)")
    continue
}
```

This follows Apple's official StoreKit 2 best practices from [WWDC 2021
- Meet StoreKit
2](https://developer.apple.com/videos/play/wwdc2021/10114/):

> When checking if a product is purchased, ensure that the transaction's
`revocationDate` equals nil (to exclude refunded transactions) and that
subscriptions where a customer has upgraded don't have the `isUpgraded`
flag set to true.

**Apple Documentation:**
-
[`Transaction.isUpgraded`](https://developer.apple.com/documentation/storekit/transaction/3812954-isupgraded)
- A Boolean that indicates whether the user upgraded to another
subscription
-
[`Transaction.revocationDate`](https://developer.apple.com/documentation/storekit/transaction/3818605-revocationdate)
- The date that the App Store refunded the transaction

## Changes

- Added `transaction.isUpgraded` check in `startTransactionListener()`
- Combined with existing `revocationDate` check for comprehensive
filtering
- Added debug logging for skipped transactions

## Testing

- ✅ All unit tests passing (10 tests)
- ✅ Swift build successful
- ✅ Maintains existing functionality (subscription renewals, duplicate
prevention)

Author: hyochan

Sources/OpenIapModule.swift

@@ -838,9 +838,9 @@ public final class OpenIapModule: NSObject, OpenIapModuleProtocol {
                     let transaction = try self.checkVerified(verification)
                     let transactionId = String(transaction.id)
 
-                    // Skip revoked transactions (happens during subscription upgrades)
-                    if transaction.revocationDate != nil {
-                        OpenIapLog.debug("⏭️ Skipping revoked transaction: \(transactionId)")
+                    // Skip revoked or upgraded transactions (happens during subscription upgrades)
+                    if transaction.revocationDate != nil || transaction.isUpgraded {
+                        OpenIapLog.debug("⏭️ Skipping revoked/upgraded transaction: \(transactionId)")
                         continue
                     }