docs: document package KV cache effects

packages/web/web-search-perplexity/README.md

@@ -33,12 +33,16 @@ This is an **implementation** package: it registers a provider into `ctx.web`, i
 
 **Token effect**: Separate provider tokens are incurred per search; `maxTokens` caps the generated answer.
 
+**KV Cache effect**: Independent of the conversation request cache. An identical query under the same model route may reuse provider cache; a changed query or route establishes a different prefix.
+
 ### Conversation tool result, indirectly
 
 **What the model sees**: Through [`dsh-tool-web`](../tool-web/README.md), the conversation model sees the generated answer plus structured result metadata or URL-only citations. This provider's exact failures are `Perplexity search aborted`, `Perplexity search request failed: <error>`, and `Perplexity returned an unprocessable response body: <error>`; HTTP failures preserve the provider message. The consumer owns the error wrapper.
 
 **Token effect**: Zero direct conversation tokens from registration. Answer and source tokens are data-dependent, source count is seam-bounded, and the retained result or error is resent until compaction.
 
+**KV Cache effect**: Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.
+
 ## Known Limitations and Deferred Work
 
 - **Citation-fallback sources are URL-only** — when Perplexity omits structured `search_results[]`, sources carry no `title`/`snippet`/`publishedAt`, so the tool renders bare hostname labels.