Rhythm
Not a calendar. A calendar as input would make this a configuration app — cadence is derived, like every other rule.
S
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6 weeks · spend density · 40px of signal instead of a month grid
Friday is fastfood night · 5 of 6 weeks · $32.40 ± $10.69 · never discussed
habitweekly · Friday · variance 33% · confidence 75%
One dimension, two payloads. A commitment (fixed amount, fixed interval) leaves the anomaly set and turns on price-creep detection, which is what stops the Reality check screaming about a subscription every month. A habit catches the $71 Tuesday pizza that sits below every approval threshold.