Which I subsequently
elaborated on today. The point about gas is why it is impossible to have sharded validation even if the scripts can otherwise be guaranteed to respect their partitioning.
Gas is paid by the invoker of the script, you are aware of that?
That doesn't matter (or actually it makes the problem I explained more obvious). The script validation and the gas spend confirmation must be atomic, which means they must be confirmed in the same partition, else validation can not be partitioned per the logic I already explained.