FrequencyManager class

Applies frequency capping for natively-rendered campaigns (nudge, survey, and guide). Dart port of Android's FrequencyManager: wraps the pure FrequencyEvaluator with persistence (one FrequencyState per campaignKey) and pulls the current session id so session windows reset exactly when the session rotates (cold start / idle timeout / user change).

Persists via LocalStorage (SharedPreferences), whose reads are synchronous once initialized — so the route-time gate (isAllowed) stays synchronous. Writes are fire-and-forget; SharedPreferences updates its in-memory cache synchronously, so a later read sees the new value at once.

sessionIdProvider returns the current session id; clock is injectable for tests and defaults to wall-clock.