Function getSegmentAtTime

Returns the segment whose range covers time (startTime <= time < endTime), or — when affordance > 0 — the next segment if time falls within affordance seconds before its startTime. Returns null if neither applies.

Note: with affordance > 0, when time lies in the last affordance seconds of a segment that does cover it, the next segment is returned instead. This is deliberate — callers use affordance to absorb sub-second boundary differences across qualities — but it means the result is not strictly the segment containing time. Callers needing exact containment should pass affordance = 0.