NavigationProjection

Everything about the navigation position that is a pure function of the back stack, the active modal contexts, and the registered graph definitions.

The reducer computes one of these per state change and io.github.syrou.reaktiv.navigation.NavigationState exposes each field through an accessor of the same name, so callers read navState.visibleLayers rather than navState.derived.visibleLayers.

Adding a derived value means adding it here and adding one accessor, rather than threading a field through the state class, an intermediate holder, and every construction site.

The constructor is internal on purpose: a projection is something the reducer computes from the back stack, never something a caller supplies. Nothing outside the navigation module can fabricate one, so a derived value cannot disagree with the stack it describes.