`collectorAudiences` was a list of URLs services contributed so the
collector's client could be registered for them. Slice C needs the same
URLs as scrape jobs, and a job needs a name the audience list has no
room for — so services would have contributed to two options that must
agree.
They now contribute `publishedScrapeTargets` once, as `<job> = "<url>"`,
and the client's audiences derive from it. The drift that would have
needed maintaining is gone, and its failure mode was the quiet one: a
target whose audience was forgotten authenticates against nothing and
reads as a broken scrape rather than a missing registration.
Adds an assertion for the one collision the module system cannot catch.
Two definitions of the same key within one option are already refused
(measured); across the two scrape options nothing arbitrates, and both
entries would render into a single scrape_configs list under one
job_name.