It's pretty easy to implement, it turns out!  A new event gets sent to
the protocol TX thread when the PE is starting an AMS.  Just before the
message is transmitted, the TX thread checks if that event was received,
and if it was, it waits for the Type-C Current to look like 3.0 A
(SinkTxOk) before proceeding.  Now, to the best of my knowledge, we're
actually doing Power Delivery 3.0 as correctly as we ever did Power
Delivery 2.0, which is to say, everything is right except we don't do
BIST.