Function build_forced_compression_candidate_plan 

pub fn build_forced_compression_candidate_plan(
    session: &Session,
    model_name: &str,
    configured_budget: Option<&TokenBudget>,
    summary_target_ratio: f64,
    trigger_type: CompressionTriggerType,
) -> Result<CompressionCandidatePlan, CompressionPlanError>

Select the exact archive candidates before invoking the summarization model.

Candidate segments are considered oldest-first. Generic tool chains remain atomic because selection operates on crate::MessageSegmenter output; newest user turns, never_compress, and skill chains remain active.