Move SDF precomputation to the mujoco compiler.

Before:
```
 Simulation time      : 2.24 s
 Steps per second     : 4472
 Realtime factor      : 8.94 x
 Time per step        : 223.6 µs

 Newton iters / step  : 2.47
 Contacts / step      : 3.37
 Constraints / step   : 13.49
 Degrees of freedom   : 12
 Dynamic memory usage : 0.2% of 14M
```

After:
```
 Simulation time      : 1.71 s
 Steps per second     : 5854
 Realtime factor      : 11.71 x
 Time per step        : 170.8 µs

 Newton iters / step  : 2.19
 Contacts / step      : 3.45
 Constraints / step   : 13.79
 Degrees of freedom   : 12
 Dynamic memory usage : 0.2% of 14M
```

PiperOrigin-RevId: 781075182
Change-Id: Ie509047ff581ab0df4b10bbd394c27d350ab5a13

src/user/user_objects.h

@@ -222,6 +222,7 @@ struct mjCOctree_ {
   std::vector<int> child_;           // children of each node     (nnode x 8)
   std::vector<double> node_;         // bounding boxes            (nnode x 6)
   std::vector<int> level_;           // levels of each node       (nnode x 1)
+  std::vector<double> coeff_;        // interpo coefficients      (nnode x 8)
   std::vector<Triangle> face_;       // mesh faces                (nface x 3)
   double ipos_[3] = {0, 0, 0};
   double iquat_[4] = {1, 0, 0, 0};
@@ -246,6 +247,8 @@ class mjCOctree : public mjCOctree_ {
     level_.clear();
     face_.clear();
   }
+  void AddCoeff(double coeff) { coeff_.push_back(coeff); }
+  const std::vector<double>& Coeff() const { return coeff_; }
 
  private:
   void Make(std::vector<Triangle>& elements);