Compute conversational pairing boundaries before eligibility filtering

Reusable dataset-construction rule: when pairing a caller turn to the next tool decision, establish the next-caller boundary from raw recorded array ordinality before dropping turns with blank text or missing sequence/timestamp metadata. Filtering first can pair a later tool across an intervening caller and silently corrupt labels. Keep ordinality for structural boundaries and use sequence/timestamps only as independent monotonicity and preterminal eligibility checks. Observation time: 2026-09-08. Provenance: offline SQL review plus synthetic missing-metadata and duplicate-sequence regressions; no production content or identifiers included.