Applies a derived plugin to a target project.
mix capstone.plugin.apply cache ../my_appReads priv/meta/meta_<name>/, renders it against the target's OWN name
triple, and writes each entry according to its ownership mode. Run
mix capstone.check afterwards: an entry whose anchor could not be located
lands as a conflict region that will not compile until a human resolves it.
A target holding a target.exs also gets a plugin.exs recording what
was written and with which content hashes. One without is installed all the
same, and the task says so.