Rejigged solver statistics in preparation for islanding.

- See public description below.
- Stopped incrementing the iteration count in saveStats().
- `sizeof(mjSolverStat) == 40`, so this ends up costing 160KB, up from 40KB.

BEGIN_PUBLIC

Changed the size of `mjData.solver`, the structure used to collect solver diagnostic information. The array is now of length `mjNISLAND * mjNSOLVER`, where each row of length `mjNSOLVER` contains separate solver statistics for each constraint island. Until solver islanding is implemented, only row 0 is used.

- The new constant `mjNISLAND` is set to 20.
- `mjNSOLVER` is reduced from 1000 to 200.
- Added `mjData.solver_nisland`, the number of islands for which the solver ran.
- `mjData.solver_niter` (renamed from mjData.solver_iter) and `mjData.solver_nnz` are now integer vectors of length `mjNISLAND`.

END_PUBLIC

PiperOrigin-RevId: 565030093
Change-Id: I773e918805c6ced79f0dab5f19ea23956760c8c5

include/mujoco/mjxmacro.h

@@ -636,8 +636,7 @@
     X( size_t,    maxuse_arena       ) \
     X( int,       maxuse_con         ) \
     X( int,       maxuse_efc         ) \
-    X( int,       solver_iter        ) \
-    X( int,       solver_nnz         ) \
+    X( int,       solver_nisland     ) \
     X( int,       nbodypair_broad    ) \
     X( int,       nbodypair_narrow   ) \
     X( int,       ngeompair_mid      ) \
@@ -654,12 +653,14 @@
 
 
 // vector fields of mjData
-#define MJDATA_VECTOR                                   \
-    X( mjWarningStat,  warning,        mjNWARNING,  1 ) \
-    X( mjTimerStat,    timer,          mjNTIMER,    1 ) \
-    X( mjSolverStat,   solver,         mjNSOLVER,   1 ) \
-    X( mjtNum,         solver_fwdinv,  2,           1 ) \
-    X( mjtNum,         energy,         2,           1 )
+#define MJDATA_VECTOR                                              \
+    X( mjWarningStat,  warning,           mjNWARNING,  1         ) \
+    X( mjTimerStat,    timer,             mjNTIMER,    1         ) \
+    X( mjSolverStat,   solver,            mjNILSAND,   mjNSOLVER ) \
+    X( int,            solver_niter,      mjNISLAND,   1         ) \
+    X( int,            solver_nnz,        mjNISLAND,   1         ) \
+    X( mjtNum,         solver_fwdinv,     2,           1         ) \
+    X( mjtNum,         energy,            2,           1         )
 
 
 // alias XMJV to be the same as X