apply_drawdown_killswitch: flatten the book once running drawdown exceeds kill_dd
(default 15%), re-enter only after recovery inside reenter_dd (7%) hysteresis. The
drawdown signal tracks un-throttled equity so recovery is observable while flat; output
is the throttled series; no lookahead (each day uses prior-day kill-state). The small-
capital tail control - survive a crypto-deleveraging by standing down, not by paying for
a hedge sleeve. Pure + composable overlay on combine_book output. Test + mypy green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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