opencl: port fused ssm_scan kernel (Mamba-2, d_state in {128, 256}) to GPU (#26439)

* opencl: port fused ssm_scan kernel (Mamba-2, d_state in {128, 256})

Fold the fused per-token SSM_SCAN recurrent step from opencl/gdn-qwen36-35b
onto the unified base. Previously SSM_SCAN fell back to CPU here; now scalar-A
Mamba-2 with d_state in {128,256}, all-f32, runs on GPU. Other shapes (incl.
Mamba-1 element-wise A) still fall back. test-backend-ops -o SSM_SCAN passes on
Adreno X2-90. opt-out via GGML_OPENCL_DISABLE_SSM_SCAN=1.

* opencl: cleanup

* opencl: require K == 1

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Co-authored-by: Li He <lih@qti.qualcomm.com>