Function wait_until_ready 

async fn wait_until_ready(state: &Arc<ServerState>) -> Result<(), ErrorEnvelope>

Wait until the worker slot is Ready, kicking a single-flight background boot if Empty. Waits up to warming_deadline (in ≤250 ms slices, woken early by boot_done); a boot longer than the budget returns WORKER_WARMING. Holds the slot lock only for the instantaneous state checks, never across the wait, and never while holding a permit (R2 seat-A).