Module spawn_decay 

Continuous spawn/decay α ramps (P3).

Replaces binary temporal on/off with fade-in/out over valid-time windows.

Functions

spawn_decay_alpha

Compute the visibility α ∈ [0, 1] for an asset at now given its valid-time interval and optional ramp durations (seconds).

temporally_active

Whether an asset should be considered visible at all (α > 0).