v0.2.1

Prompt-cache-safe rewriting. A default-on cache guard decides each tool-output block's wire form
on first sight and freezes it, never rewriting a block a provider has already cached, so the
request prefix stays byte-stable and prompt caching holds.

A fresh block offloads only when its preview covers the content; otherwise it stays verbatim, so a
just-arrived result is never forced through a resolve. Relevance split runs only with the guard off.

New SW_CACHE_GUARD and SW_HOLD_TURNS host controls, plus a prompt-cache benchmark and a
live-provider probe.