Function residual_add_rms_norm 

pub fn residual_add_rms_norm(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    hidden: DevicePtr,
    src: DevicePtr,
    weight: &DenseWeight,
    output: DevicePtr,
    residual: DevicePtr,
    num_tokens: u32,
    hidden_size: u32,
    eps: f32,
    stream: u64,
) -> Result<()>

Fused residual add + RMS norm + residual save.

hidden[i] += src[i]; normed = rms_norm(hidden) * (1+weight); residual = hidden. Eliminates one kernel launch per fusion site (48 per decode step).

Kernel: residual_add_rms_norm(hidden, src, weight, output, residual, hidden_size, eps) Grid: (num_tokens, 1, 1) Block: (min(hidden_size, 1024), 1, 1)