LEVEL event: price is currently inside or outside a band — true every bar the condition holds. USE WHEN gating (e.g. 'while price is outside upper Bollinger, suppress new entries'). DO NOT USE for one-shot triggers on band breach — use 'band_edge' for the touch / cross_in / cross_out edges. Level counterpart to 'band_edge'. Example band: {"band":"bollinger","length":20,"mult":2}