ggml webgpu: add CEIL operation support (llama/18605)

ggml webgpu: add CEIL operation support (llama/18605)

* ggml-webgpu: add CEIL operation support

      Add support for the CEIL unary operation in the WebGPU backend:
      - Add CEIL_FUNC shader template in unary_op.wgsl
      - Add 4 shader variants (f32, f16, inplace versions)
      - Initialize CEIL pipelines in ggml-webgpu.cpp
      - Register CEIL in supports_op function

* docs: update WebGPU ops support for CEIL