/// Snapshot of lightweight cache counters.
///
/// The counters are intentionally lightweight and approximate enough for local
/// observability. They are not intended to be a durable metrics store.
///
/// # Example
///
/// ```rust
/// use hydracache_core::CacheStats;
///
/// let stats = CacheStats::default();
/// assert_eq!(stats.hits, 0);
/// assert_eq!(stats.single_flight_joins, 0);
/// ```