Task 0548 / CAP-85 (protocol 28): the contract this one borrows its code
from, and the tag naming which of that owner's executables it runs. Both
null for every contract that carries its own executable.
When they are set, wasm_hash above is the hash the reference resolves
to at the time of the request — the code the contract genuinely runs,
which is what the protocol's own get_address_executable reports too.
It is NOT stored against this contract: the owner can re-point the whole
fleet without a single ledger change touching it, so a stored copy would
go quietly out of date. Read these two to tell "runs its own code" from
"runs someone else's".