Make constraint island discovery on by default.

Also fix latent bug in mjData serialization.

PiperOrigin-RevId: 799505349
Change-Id: I299e7cc8133fa2ec0b339a7ff3d02543e06778b1

src/engine/engine_forward.c

@@ -723,7 +723,7 @@ void mj_fwdConstraint(const mjModel* m, mjData* d) {
   mju_zeroInt(d->solver_niter, mjNISLAND);
 
   // check if islands are supported
-  int islands_supported = mjENABLED(mjENBL_ISLAND)      &&
+  int islands_supported = !mjDISABLED(mjDSBL_ISLAND)    &&
                           nisland > 0                   &&
                           m->opt.noslip_iterations == 0 &&
                           (m->opt.solver == mjSOL_CG || m->opt.solver == mjSOL_NEWTON);

src/engine/engine_island.c

@@ -411,7 +411,7 @@ void mj_island(const mjModel* m, mjData* d) {
   int nv = m->nv, nefc = d->nefc, ntree=m->ntree;
 
   // no constraints: quick return
-  if (!mjENABLED(mjENBL_ISLAND) || !nefc) {
+  if (mjDISABLED(mjDSBL_ISLAND) || !nefc) {
     d->nisland = d->nidof = 0;
     return;
   }
@@ -454,7 +454,7 @@ void mj_island(const mjModel* m, mjData* d) {
     return;
   }
 
-  // count ni: total number of dofs in islands
+  // count nidof: total number of dofs in islands
   int nidof = 0;
   for (int i=0; i < nv; i++) {
     nidof += (tree_island[m->dof_treeid[i]] >= 0);

src/engine/engine_support.c

@@ -62,7 +62,8 @@ const char* mjDISABLESTRING[mjNDISABLE] = {
   "Midphase",
   "Eulerdamp",
   "AutoReset",
-  "NativeCCD"
+  "NativeCCD",
+  "Island"
 };
 
 
@@ -72,8 +73,7 @@ const char* mjENABLESTRING[mjNENABLE] = {
   "Energy",
   "Fwdinv",
   "InvDiscrete",
-  "MultiCCD",
-  "Island"
+  "MultiCCD"
 };