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: its time map read with the reverse and ping-pong mirror of one project frame at frame_rate, exactly as evaluation draws it.

Editors that key a clip’s source composition, or show where a clip reads its source, take the time from here. tests/golden/time-rules-corpus.json pins it for the TypeScript port.

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