Fix type errors in engine and plugins.

- Preparation for a float32 build of MuJoCo.
- Fix use of `float`-typed `fabs` in `mju_eig3`.

PiperOrigin-RevId: 643804942
Change-Id: I89f64e8fd39f4e70e78d7607816283e217912383

include/mujoco/mjplugin.h

@@ -99,7 +99,7 @@ struct mjpPlugin_ {
   void (*copy)(mjData* dest, const mjModel* m, const mjData* src, int instance);
 
   // called when an mjData is being reset (required)
-  void (*reset)(const mjModel* m, double* plugin_state, void* plugin_data, int instance);
+  void (*reset)(const mjModel* m, mjtNum* plugin_state, void* plugin_data, int instance);
 
   // called when the plugin needs to update its outputs (required)
   void (*compute)(const mjModel* m, mjData* d, int instance, int capability_bit);