receiveSignalEvents blocks in WaitStreamConnected until the signal stream
connects or the context is cancelled. If Stop cancelled e.ctx while Start
was parked there, Start kept going: it started the remaining subsystems on
a cancelled context and marked a shutting-down engine as started. Return
the context error from receiveSignalEvents and propagate it from Start, so
the deferred cleanup runs and the cancellation reaches the caller.
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