Function spawn 

pub fn spawn(
    pool: &ConnectionPool,
    capacity: usize,
) -> Result<WriterTaskHandle, SqliteError>

Spawn the write-owner task (ADR-067 Component A) on the current Tokio runtime.

Opens a dedicated standalone writer connection (ConnectionPool::open_standalone_writer) that the task owns exclusively for its lifetime — independent of the pool’s Mutex-guarded writer() connection, which unmigrated paths continue to use in this slice. Returns the cloneable WriterTaskHandle sender half; the task keeps running in the background until every clone of the handle is dropped and the channel closes, at which point the drain loop exits.

capacity bounds the channel (ADR-067 recommends 256; PoolConfig::write_queue_capacity resolves the default from KHIVE_WRITE_QUEUE_CAPACITY). Slice 1 commits one request per BEGIN IMMEDIATE — Component B’s batched-commit window is a later slice.

Must be called from within a Tokio runtime context (this calls tokio::spawn). Returns an error if the pool cannot open a standalone writer connection — for example, an in-memory pool, which has no standalone-connection support (ConnectionPool::open_standalone_writer).