Function cache_symbolic_data_off_runtime 

async fn cache_symbolic_data_off_runtime(
    cache: Arc<DiskCache>,
    cache_key: String,
    symbolic: SymbolicDataWithProvenance,
)

Write the symbolic sidecar through cache_keys::cache_symbolic_data on the blocking pool (#4696).

That call JSON-encodes the whole 2D symbol stream before it writes, and awaited on an async worker the encode holds that worker for its whole length. Every parse route writes the sidecar through here. The write itself is async, so the blocking thread drives it with Handle::block_on.