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-device.cpp

@@ -1834,6 +1834,23 @@ ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_col2im_1d(ggml_m
     return res;
 }
 
+ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_snake(ggml_metal_library_t lib, enum ggml_type type) {
+    GGML_ASSERT(type == GGML_TYPE_F32 || type == GGML_TYPE_F16 || type == GGML_TYPE_BF16);
+
+    char base[256];
+    char name[256];
+
+    snprintf(base, 256, "kernel_snake_%s", ggml_type_name(type));
+    snprintf(name, 256, "%s", base);
+
+    ggml_metal_pipeline_with_params res = ggml_metal_library_get_pipeline(lib, name);
+    if (!res.pipeline) {
+        res = ggml_metal_library_compile_pipeline(lib, base, name, nullptr);
+    }
+
+    return res;
+}
+
 ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_conv_transpose_2d(ggml_metal_library_t lib, const ggml_tensor * op) {
     assert(op->op == GGML_OP_CONV_TRANSPOSE_2D);