sample/compile cannot load USD decoder plugins without manual plugin registration

### The feature, motivation and pitch

`sample/compile` now uses the generic `mj_parse` / `mj_encode` path, which is great for decoder-backed formats. However, decoder plugins still need to be registered before `mj_parse` can use them.

In local USD roundtrip testing, this means the USD decoder plugin is built but `compile model.usda out.xml` does not work unless a separate helper manually loads `libusd_decoder_plugin.so`.

It would be useful for the sample command to load plugin libraries from the standard local build/install plugin directory before parsing. This keeps the sample aligned with the decoder-plugin architecture and gives users a simple command-line path for decoder-backed formats such as USD.

### Alternatives

_No response_

### Additional context

_No response_