CentroidIndexRecord.cycle_index decoded but unused for QTOF precursor linking

[Nathan-D-R]

Found during the 2026-07-24 cross-repo parity sweep.

crates/openszraw/src/raw/qtfl.rs:33 documents cycle_index as a verified per-acquisition-cycle counter, but reader.rs's qtfl_spectra (lines 372-433) links each MS2 scan to a precursor purely via a positional "most recently seen MS1" heuristic (last_ms1_native_id, line 395/423) and never reads cycle_index at all (confirmed via grep, zero other references). The more authoritative field is already decoded and sitting unused.

Effort: small (validate/tie-break using cycle_index). Severity: low-medium - correctness risk only if scan order/interleaving ever deviates from strict cycle grouping.