* metal: fuse snake activation (mul, sin, sqr, mul, add)
Mirror the CUDA, Vulkan and CPU snake fusion: same matcher on the naive
5-op chain, same F32 contract on a and inv_b, same F32/F16/BF16 kernel
with F32 compute. Follows the Metal backend idioms: bf16 instantiation
gated behind GGML_METAL_HAS_BF16 and concurrency ranges checked on the
remaining chain nodes before encoding, as done by the bin fusion.
Covered by the existing backend-agnostic SNAKE_FUSE tests.
* metal: absorb snake fusion into ggml_metal_op_bin
Extract the matcher to ggml_metal_op_can_fuse_snake, mirroring the
Vulkan naming, and dispatch the fused path from ggml_metal_op_bin.
The encode loop switch is back to a single call per case.
Address review from ggerganov
* metal: fix indentation in ggml_metal_op_can_fuse_snake
e376ba3ad8