mlx: add compiled closure support

Wraps MLX's mlx_compile API so Go functions can be traced into fused
kernels. Contiguous elementwise chains collapse into a single
Metal/CUDA kernel instead of launching one per op.

Exposes Compile plus arity helpers (Compile1/2/3) that mirror Python's
@mx.compile decorator shape, lazily building the closure on first call
so package-level declarations work before the MLX dylib loads.

x/mlxrunner/mlx/array.go

@@ -27,7 +27,11 @@ var arrays []*Array
 
 func New(name string) *Array {
 	t := &Array{name: name}
-	arrays = append(arrays, t)
+	if tracing {
+		traceScratch = append(traceScratch, t)
+	} else {
+		arrays = append(arrays, t)
+	}
 	return t
 }