Function prepare_cleared_turns_summary 

pub fn prepare_cleared_turns_summary(
    msgs: &[ChatMessage],
    policy: &ReductionPolicy,
    prior: &ReductionLog,
    summarizer: &dyn SpanSummarizer,
) -> Option<PreparedClearSummary>

TR-7’s one side-call site (SPEC.md: “an explicit, budgeted, injectable side-call… never blocking the main loop”). Call this against the exact same msgs slice about to be projected (mirrors probe_read_freshness’s own calling convention), thread the result through ReductionPolicy::cleared_turns_summary before calling project_messages. Only ever does anything when policy.summarize_cleared_turns is set; callers that never enable TR-7 can skip calling this entirely (dev/01: project_messages behaves identically either way when the policy gate is off).

Returns None — meaning project_messages will render the deterministic stub — whenever: TR-7 is off; a TurnsCleared range is already established (A10 fires at most once per session, singleton, so no side-call is ever needed for an already-decided span); no clear range currently triggers; the candidate span is below the cost-guard floor (dev/05, no side-call attempted at all); or the summarizer itself errors or returns an empty/blank result (dev/03’s fault-injection contract — this NEVER propagates an error to the caller, by design).