The broadcast upload extension never appeared in the iOS broadcast picker
(or Control Center screen-recording list) because its Info.plist set
RPBroadcastProcessMode to the invalid string "BroadcastUpload". iOS only
recognises "RPBroadcastProcessModeSampleBuffer" for a sample-buffer upload
extension and silently rejects any other value, hiding the extension.
Also sign VoiceCatCore.xcframework in build-xcframework.sh: Xcode 15+ rejects
unsigned binary XCFrameworks consumed as SwiftPM binary targets. Signs with
CODESIGN_IDENTITY, else the first Apple Development identity, else ad-hoc.