Per-event Wiener-α=0.4 EMA on |Δmid| with first-observation bootstrap. Floor source for the SL/trail-distance controller (spec §6). Thread 0 of the broadcast-snapshot kernel handles the update per backtest; threads 1..9 still handle the 10 level writes. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>