TransactionCoordinator class

Bridges the Dart MobileReaderSdk.startTransaction flow to the gateway-handoff contract.

What it does.

• Wraps MobileReaderSdk.startTransaction(request) (the native runtime's encrypted-payload capture path) and MobileReaderSdk.events (the unified event stream that delivers encryptedPayloadReady).
• In GatewayMode.payloadOnly, returns a TransactionCoordinatorOutcome.payloadCapturedOnly wrapping the native result (status=unknown); the POS forwards the encrypted payload via its own backend.
• In GatewayMode.gatewayHandoff, builds a GatewayTransactionRequest from the encrypted-payload event, calls GatewayHandoffOrchestrator.handoff, and maps the GatewayHandoffResponse into a typed TransactionCoordinatorOutcome.

What it does NOT do.

• It does not call em-api. It does not call Magensa. It does not decrypt anything. It does not synthesize approvals.
• It does not modify the native runtime - it consumes whatever the existing platform-channel adapter surfaces.
• It does not log encrypted payload bytes, KSN values, bearer tokens, PAN, track, CVV, or ARQC values. The wrapped types' toStrings already redact; this layer adds no logging of its own.

Concurrency. One coordinator runs one transaction at a time. A second concurrent runTransaction call resolves immediately as a readerFailed outcome wrapping a ReaderError(transactionInProgress).

Lifecycle. dispose() releases the orchestrator's resources (if any) and cancels the events subscription. The wrapped MobileReaderSdk is owned by the POS and is NOT torn down here.