prompt_hard.md

Add distributed tracing with context propagation across the library. Every public method must create a child span under the caller's trace context if one is provided. Spans must record the cache name, key, outcome, and byte sizes as attributes. Async eviction must propagate the originating trace context into its background work. The tracing backend must be swappable (i.e. the library must not hard-depend on any specific tracing SDK). Trace context must flow correctly across actor hops without leaking between unrelated concurrent operations.