Function merge_stage 

pub fn merge_stage(
    pending: &mut ChunkStage,
    new: ChunkStage,
    budget_chunks: usize,
)

Merge a newly staged batch into a still-pending one instead of replacing it.

The main thread stages every frame while streaming, but the render thread consumes asynchronously; a plain stage = Some(new) DROPPED any unconsumed batch — those chunks were already marked clean in the cache and kept their slots, so they were drawn forever with uninitialized pool memory (the giant garbage-triangle walls). Realize work is cumulative (descriptors + surface patches + scatter aux append); the instance set, surface globals and scatter visible/params are snapshots (latest wins). If the merged batch exceeds budget_chunks (render thread stalled for hundreds of frames), the OLDEST realizes are dropped — they re-dirty naturally.