Constant EVENT_EXTANT_VALUE 

pub const EVENT_EXTANT_VALUE: &[u8];

Singleton value for the internal IndexDimension::EventExtant marker: every real event maps to the same key. It carries a single placeholder byte so the encoded row key has the standard [tag, value...] shape and passes BitmapKey::new’s length invariant (dimension keys must be at least two bytes) without a per-tag carve-out. The marker is a singleton — one row per bucket, not per event — so the extra byte costs ~1 byte per bucket-row.