A conversion answers with the oldest rate in force

RateBook.rate_for in src/ledgerlite/fx.py answers one question: which rate

of a currency was in force at a moment. The rule, as documented in the module:

• a rate is in force from the instant it was published until the next one is;
• the rate published most recently at or before the moment is the one in

force;

• a rate published later is knowledge the ledger did not have, so it is never

the answer, and there is no answer before the first publication.

When a currency's rate is revised, the old rate stops being the answer from the

revision onward. The defect makes the book answer with the first rate it can

find at or before the moment, so after a revision every conversion is silently

valued at the superseded rate — no error, just wrong numbers.

The repository's own suite is green on the pinned commit, because it publishes

a single rate per currency, for which "first in force" and "last in force" are

the same rate. The defect only shows up once a currency has more than one

published rate.