Load ExtensionKit extensions from Extensions/ as well as PlugIns/

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Problem

AppBundle.loadExtensions() only looks in PlugIns/. ExtensionKit extensions (EXAppExtensionAttributes, product type com.apple.product-type.extensionkit-extension) are embedded in Extensions/ instead. One example is the iOS 26.1/27 PhotoKit background upload extension (com.apple.photos.background-upload).

Because SideStore builds everything from appExtensions (App IDs, profiles, bundle ID rewriting, entitlements, signing), such an extension is skipped. It keeps its original bundle ID, gets no profile, and is signed with the main app's entitlements, so the install fails.

Moving the extension to PlugIns/ is not a workaround. iOS then rejects it with IXErrorDomain Code=17 "extensionDictionary must be set in placeholder attributes for an app extension placeholder", because it expects an NSExtension dictionary there.

Fix

Scan both PlugIns/ and Extensions/ for .appex bundles. Apps without an Extensions/ folder behave exactly as before.

Testing

• New unit test appExtensionsIncludeExtensionKitExtensions (a fake app with one extension in each folder). swift test passes on macOS 26 / Xcode 26.4, all 6 tests. Added import Foundation to the test file, which the new test needs.
• On a device: SideStore develop (12ff39c) with this change, a free developer account, and iOS 27. An app with a PhotoKit background upload extension installs, the extension gets its own App ID, profile and app group, and iOS runs it to upload photos in the background.

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