With authored platforms.<plat>.extra outranking the <plat>: block, a user
could replace a key the administrator pinned in the managed block
(require_mention, allow_from, ...) by writing it under their own
platforms.<plat>.extra. On main the managed block value wins.
Drop managed block-pinned keys from the authored snapshot. A key the
managed layer sets under platforms.<plat>.extra stays authored, so it now
also beats a user's top-level block.
77bc57df41