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

python/mujoco/mjdata_meta.h

@@ -15,6 +15,7 @@
 #ifndef MUJOCO_PYTHON_MJDATA_META_H_
 #define MUJOCO_PYTHON_MJDATA_META_H_
 
+#include <mujoco/mujoco.h>
 #include <mujoco/mjxmacro.h>
 #include "raw.h"
 #include "util/crossplatform.h"
@@ -74,6 +75,8 @@ struct MjDataMetadata {
   MJDATA_METADATA
 #undef X
 
+  bool is_dual;
+
  private:
   MjDataMetadata() = default;
   MjDataMetadata(const MjDataMetadata& other) = default;
@@ -94,10 +97,7 @@ struct MjDataMetadata {
 
         MJDATA_METADATA
 #undef X
-        dummy_() {}
-
-  // Dummy variable to terminate X macro sequences.
-  MUJOCO_MAYBE_UNUSED bool dummy_;
+        is_dual(mj_isDual(m)) {}
 };
 
 }  // namespace mujoco::python