Function reshape_and_cache_flash_v_only 

pub fn reshape_and_cache_flash_v_only(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    value: DevicePtr,
    v_cache: DevicePtr,
    slot_mapping: DevicePtr,
    num_tokens: u32,
    num_kv_heads: u32,
    head_dim: u32,
    block_size: u32,
    value_stride: u32,
    stream: u64,
) -> Result<()>

V-only paged cache write — companion to the fused K-path so the K side of the cache stays exclusively owned by fused_k_norm_rope_cache_write_*. Use this when the fused K kernel is active to avoid the existing reshape_and_cache overwriting the correct K values with a double-rounded copy.