cuda: align snake fusion matcher with the other backends (llama/25460)

cuda: align snake fusion matcher with the other backends (llama/25460)

* cuda: fix snake fusion type predicate, a and inv_b are F32

The matcher required a->type == x->type while launch_snake reads both
as const float *, matching the CPU and Metal contract where a and inv_b
stay F32. F16/BF16 chains never fused and fell back to the naive path,
and a hypothetical all F16 chain would have read F16 bits as float.
Aligns the predicate and the comment with ggml-cpu.c

* cuda: reject snake fusion on non-contiguous operands

The kernel reads x[idx] and a[c] / inv_b[c] linearly, so a
non-contiguous view passing the matcher would silently read wrong data.
Mirror the contiguity guard already present in the CPU, Vulkan and
Metal matchers.