Function cache_churn_notice 

pub fn cache_churn_notice(server: &str, tool_count: usize) -> String

P5-2 (§2.2 C2 “connect invalidates cache prefix”; §2 module 25 cache is the referee): the churn notice a caller (crates/cli’s attach_mcp) emits when connecting a server under an active CachePlan::ImportedPrefix — a pure, independently-testable function so the wording/threshold logic isn’t buried in CLI plumbing. “At minimum emit the churn signal” (P5-2 build brief) — this is that signal; it does not itself reset any cache bookkeeping (see Agent::register_tool’s own C2 note for the runtime half: any tool registered after the agent’s first turn resets cache_established, MCP-sourced or not).