Dynamically allocate contact and efc_ arrays on a new memory arena.

- Add private function `mj_arenaAlloc`. This is used internally to allocate memory from the arena.

- Add private function `mj_nefc` to count constraints. This function returns a tight upper bound on `d->nefc`. The number of counted constraints can be slightly bigger than exact `d->nefc` in the case of constraints with empty Jacobian, as when placing a frictional tendon between two world sites.

- Add new `memory` attribute to the `size` XML element for specification of arena memory size. This attribute is mutually exclusive with `nstack` and `njmax` specifications, which are now deprecated (but left around for the time being for legacy compatibility).

- Move `d->stack` to the end of the new arena space. The stack now grows in reverse from the end.

PiperOrigin-RevId: 479341539
Change-Id: Ie019c202e0908577ffc6f833a37920858116f667

src/engine/engine_util_misc.c

@@ -956,11 +956,16 @@ const char* mju_warningText(int warning, int info) {
     break;
 
   case mjWARN_CONTACTFULL:
-    mjSNPRINTF(str, "Pre-allocated contact buffer is full. Increase nconmax above %d.", info);
+    mjSNPRINTF(str,
+               "Too many contacts. Either the arena memory is full, or nconmax is specified and is "
+               "exceeded. Increase arena memory allocation, or increase/remove nconmax. "
+               "(ncon = %d)", info);
     break;
 
   case mjWARN_CNSTRFULL:
-    mjSNPRINTF(str, "Pre-allocated constraint buffer is full. Increase njmax above %d.", info);
+    mjSNPRINTF(str,
+               "Insufficient arena memory for the number of constraints generated. "
+               "Increase arena memory allocation above %d bytes.", info);
     break;
 
   case mjWARN_VGEOMFULL: