Function propagate_sky_light_chunk 

pub fn propagate_sky_light_chunk(
    workset: &LightWorkset,
    edge_checks: SkyLightChunkEdgeChecks,
) -> Result<SkyLightUpdateResult, SkyLightPropagationContextError>

Seeds and propagates sky light for the center chunk of a scoped workset.

This matches ScalableLux SkyStarLightEngine.lightChunk: sky sections around non-empty sections are initialized, full skylight is propagated downward, then the caller chooses between validating edge consistency or pulling already-initialized neighbor levels inward.