Function picture_source_time 

pub fn picture_source_time(
    timeline: TimeRange,
    source: TimeRange,
    time_map: TimeMap,
    frame_rate: FrameRate,
    time: MediaTime,
) -> Option<MediaTime>

The source instant a clip’s picture shows at timeline time, mirroring by project frames at frame_rate exactly as evaluation draws it.

tests/golden/time-rules-corpus.json pins this for the TypeScript port.

Returns None for an empty range or a value that does not fit in ticks.