refactor(broadcast-channel): extract passthrough() factory, remove 20 identical handlers

refactor(broadcast-channel): extract passthrough() factory, remove 20 identical handlers

Replace 20 identical BroadcastChannel handler methods (handleAuthorize,
handleDataTransfer, handleHeartbeat, handleStatusNotification, etc.)
with a single passthrough(RequestCommand) factory method that creates
a CommandHandler forwarding the request payload to requestHandler.

Retain only handlers with custom logic: handleBootNotification (merges
bootNotificationRequest), handleMeterValues (version-specific meter
value building), handleStopTransaction (adds meterStop).

Net reduction: -208 lines.