CueSelector

The active-cue rule, as one pure function: which cues are visible at a media time.

Purity is the whole design. Because the answer depends on nothing but (cues, time), a seek needs no reconstruction machinery at all: asking for the new time IS the reconstruction, in both directions, which stateful subtitle engines get wrong. The engine keeps the sorted list and asks on every timing edge; nothing here remembers anything.

Overlap policy: every cue whose window contains the instant is active, ordered by layer then start time, so a two-speaker overlap shows both lines and the later speaker stacks above the earlier one (the layout engine stacks bottom-up in this order).