metal: fuse snake activation (mul, sin, sqr, mul, add) (#25459)

* 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

ggml/src/ggml-metal/ggml-metal-impl.h

@@ -616,6 +616,11 @@ typedef struct {
     int32_t  p0;
 } ggml_metal_kargs_col2im_1d;
 
+typedef struct {
+    int32_t T;
+    int32_t C;
+} ggml_metal_kargs_snake;
+
 typedef struct {
     int32_t  IC;
     int32_t  IH;