docs: document package KV cache effects

packages/llm/token-meter/README.md

@@ -42,6 +42,8 @@ Both plugins have usable defaults. A deployment with a different capacity config
 
 Indirectly, through consumers such as `dsh-compact-basic`; the service itself adds no prompt, message, schema, tool, or model call.
 
+**KV Cache effect**: No direct invalidation; the named consumer owns any request-prefix changes.
+
 ## Known Limitations and Deferred Work
 
 - **The fixed heuristic is approximate** — content without reusable provider usage is priced by character count plus structural overhead, not an exact provider tokenizer or request serializer.