LEVEL interval on continuous values: true every bar low <= subject <= high, with EACH bound's inclusivity set separately — low_inclusive and high_inclusive are BOTH REQUIRED, because the two two-bound sites this replaces disagree (band_level inside is exclusive on both edges, zone membership is inclusive on both), so any default would be a silent choice between them. USE WHEN: 'RSI between 30 and 70', 'price inside the Bollinger band', any half-open window like [30, 70) that a single flag cannot express. Bounds may be signal refs, not just constants, so they can be moving series. DO NOT USE for a one-sided threshold — use 'compare'. DO NOT USE for membership in a discrete set — use 'in_set'. A literal interval that is empty (low above high, or equal bounds with an exclusive end) is refused at construction rather than shipped as a strategy that never fires.