Problem: credentials lived in a top-level _credentials namespace,
requiring special preservation logic in clear_all() and a separate
key hierarchy from the platform data they belong to.
Solution: move credentials from _credentials.<platform> to
<platform>._credentials. Migrate v1 caches on load, skip underscore-
prefixed keys when enumerating contest IDs and summaries, and simplify
clear_all() now that no special preservation is needed.