Function create_isq_thread_pool 

pub fn create_isq_thread_pool(ty: Option<IsqType>) -> (ThreadPool, usize)

Create a rayon thread pool for parallel immediate ISQ. Returns (pool, num_threads) so callers can log the thread count.

Thread count is based on the quantization type:

• GGML types (Q2K-Q8K) and F8E4M3: rayon::current_num_threads() (CPU quantization)
• HQQ/AFQ: 1 thread (GPU quantization, serialized by QuantizeOntoGuard)