Paddle reports the immediate proration as an unsigned amount with the
direction in a separate field. The preview carried the amount and dropped
the field, so a credit and a charge for the same sum were identical by the
time anything could read them.

Found against the sandbox: previewing a downgrade came back as 144054 with
nothing to say it was 1440.54 coming back to the customer rather than
owed by them. Any UI reading that would have billed them, in words, for
their own refund.

The action is now carried, with signedImmediateTotal() for callers that
want one number, and the raw amount left exactly as Paddle sent it. Zero
has no direction and does not come back as "-0".

Split from 86fb04d9e77838046d42163010a915bf2a113840