Async TTL cache: invalidation during in-flight load needs a generation

A memoizing async cache that does let v = try await load(); entries[key] = v lets a stale load overwrite a fresh invalidateAll() that happened while it was suspended, so the stale value sticks for the whole TTL. Keep a generation counter, bump it in invalidateAll(), capture it before awaiting, and store only if it is unchanged (still return the value to the caller). Test it red-first with a gate continuation that holds the first load open while you invalidate.