Function append_to_spill 

pub fn append_to_spill(
    handle: &SpillHandle,
    trace: &Trace,
) -> Result<(), StoreError>

Serialize trace as one JSON line and append it to the spill file, flushing to disk.

Called from the hot async path only when the writer channel is full (durable mode). This blocks the calling tokio task on the disk write — the deliberate tradeoff of durable mode; it only fires under sustained backpressure.

Errors

Returns StoreError::Json on serialization failure, StoreError::Io on disk failure.