mlx: remove noisy error output from dynamic library loading (#14346)

The recent change in #14322 added tryLoadByName() which attempts to
load libmlxc.dylib via rpath before searching directories. This is an
optimization for Homebrew installations where rpath is correctly set.

However, when rpath isn't set (which is the common case for app bundle
installations), dlopen fails and the CHECK macro prints an error to
stderr:

  ERROR - dynamic.c:21 - CHECK failed: handle->ctx != NULL

This error is misleading because it's an expected failure path - the
code correctly falls back to searching the executable directory and
loads the library successfully. The error message causes user confusion
and makes it appear that something is broken.

Replace the CHECK macro with a simple return code so the C code fails
silently. The Go code already handles error logging appropriately:
tryLoadByName() fails silently (intentional fallback), while
tryLoadFromDir() logs via slog.Error() when explicit path loading fails.

x/mlxrunner/mlx/dynamic.c

@@ -18,7 +18,9 @@
 
 static int mlx_dynamic_open(mlx_dynamic_handle* handle, const char* path) {
     handle->ctx = (void*) DLOPEN(path);
-    CHECK(handle->ctx != NULL);
+    if (handle->ctx == NULL) {
+        return 1;
+    }
     return 0;
 }