The cache manager and the stats server each carried a hand-written parser
for the same file. When the format went to v4 for maturity tiers only the
manager was updated, so loadKnownWordsSet fell through to null and every
session reported 0 known words from an intact cache.
Move the state union, its parser, and a knownWordsFromState accessor into
known-word-cache-format and have both readers go through it. The stats
server no longer mentions a version at all, and both version switches close
with assertNever, so adding a V5 is a compile error at every consumer
instead of a silently empty result. An unparseable cache now warns rather
than reading as "no known words".
Two stats fixtures omitted refreshedAtMs/scope and only passed because the
stats reader was laxer than the manager; they now carry the fields every
cache the app writes has always had.
