Two fixes from the independent review of f043cd7.
RationalNs::checked_add cross-multiplied into u128 and then added without a check. Each
product fits, but their sum does not: two reduced fractions near the U64 maximum add to about
2^129, which panics in debug and wraps in release, after which the reduction returns a
confidently wrong rational. Every multiplication and the addition now go through a checked
path, as ipc-v1 section 2 requires, with a fixture row and a test that reaches the overflow
rather than the "does not fit U64" reduction limit.
The schema-set test checked fourteen hardcoded type names, so a new payload type could have
shipped outside contractDigest. It now iterates the readers own list, all fifty of them.
Also amends ipc-v1 section 5, dated: Worker.Acknowledge carries req-<U64> request ids, not
arbitrary Ids, because the serial watermark rule in that same section cannot reject reuse
after acknowledgment unless the acknowledged id carries its serial.
