A model that needs an externally-owned parameter injector – a registered
par-loader, e.g. a package's neural-network weight loader – flags that
injector's "<package>:<function>" name here. At solve setup only the loader
with that name runs, so an injector never overwrites an unrelated model's
par_ptr merely because it happens to be registered. The flag is stored on the
ui (a sticky item, so it survives model piping) like rxForcedPars(); set it to
NULL to clear.