Function fused_k_norm_rope_cache_write_bf16 

pub fn fused_k_norm_rope_cache_write_bf16(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    k_in: DevicePtr,
    k_norm_weight: DevicePtr,
    positions: DevicePtr,
    k_cache: DevicePtr,
    slot_mapping: DevicePtr,
    num_tokens: u32,
    num_kv_heads: u32,
    head_dim: u32,
    rotary_dim: u32,
    block_size: u32,
    rms_eps: f32,
    theta: f32,
    stream: u64,
) -> Result<()>

2026-09-25: K norm (x * rsqrt(mean(x^2) + eps) * (1 + w)), RoPE and the paged BF16 cache write in one kernel. K stays in FP32 from the BF16 load to the single BF16 rounding at the store. One block per (token, KV head) with head_dim threads; the kernel supports head_dim <= 256.