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

src/engine/engine_forward.c

@@ -501,7 +501,7 @@ void mj_fwdConstraint(const mjModel* m, mjData* d) {
     mju_copy(d->qacc, d->qacc_smooth, nv);
     mju_copy(d->qacc_warmstart, d->qacc_smooth, nv);
     mju_zero(d->qfrc_constraint, nv);
-    d->solver_iter = 0;
+    mju_zeroInt(d->solver_niter, mjNISLAND);
     TM_END(mjTIMER_CONSTRAINT);
     return;
   }
@@ -512,7 +512,7 @@ void mj_fwdConstraint(const mjModel* m, mjData* d) {
 
   // warmstart solver
   warmstart(m, d);
-  d->solver_iter = 0;
+  mju_zeroInt(d->solver_niter, mjNISLAND);
 
   // run main solver
   switch ((mjtSolver) m->opt.solver) {
@@ -532,6 +532,9 @@ void mj_fwdConstraint(const mjModel* m, mjData* d) {
     mjERROR("unknown solver type %d", m->opt.solver);
   }
 
+  // one (monolithic) island
+  d->solver_nisland = 1;
+
   // save result for next step warmstart
   mju_copy(d->qacc_warmstart, d->qacc, nv);