Add SO3 transmission and native orientation actuator.

https://youtu.be/17XpwnqyCXs

New transmission type mjTRN_SO3: a relative orientation, targeting a ball
joint or a site+refsite pair. It is the first transmission with more than
one force output: its length is the norm of the expmap vector of the
relative rotation and its moment axes are the 3 rows of the
relative rotational Jacobian, without projecting onto per-actuator gears.

New force law mjGAIN_SO3/mjBIAS_SO3: a geodesic PD servo, force =
kp * log(q_current^-1 * q_target) - kv * velocity, exact for arbitrary axis
combinations with a unique equilibrium at every commanded orientation.
Error, moment rows and velocity all live in the child frame (joint or
site): the right-difference error is the gradient of the geodesic
potential in that frame. The parent-frame (left) error is not: driving
child-frame torques with it pumps energy at large angles, settling into
steady-spinning limit cycles (the SO3LargeAngleConvergence test). The
integrator variant stores the 3D orientation setpoint in act (actnum = 3,
re-anchored to a bounded representative at integration time). Exposed in
MJCF as <orientation joint=|site=+refsite= kp kv|dampratio>, or via
<general gaintype="so3" biastype="so3">.

The setpoint input has two charts: an expmap target (3 controls, default)
or a quaternion target (4 controls) -- <orientation input="quat">, the
first actuator with different input and output widths. The signature is
recorded in a new per-actuator field actuator_ctrlspec (mjtCtrlChart),
whose meaning is scoped by the gain type the way gain/bias parameters are;
ctrlnum is derived from it at compile time and remains the layout
authority. An explicit field rather than width inference or a prm slot:
width-as-chart cannot express same-width signatures (upcoming servo input
subsets), and prm slots are the input_mode pattern this stack retires.
The force law normalizes the commanded quaternion, making it scale- and
antipodally-invariant. The all-zero ctrl still maps to the identity via
mju_normalize4, but it is a degenerate point (a nudge of any component
commands a half-turn), so quat inputs reset to the identity quaternion:
new mj_resetCtrl sets neutral ctrl values (zero, except qw = 1), called
by mj_resetData and the viewers' Clear All. The quat chart is
restricted to dyntype 'none': integrating a quaternion setpoint linearly
is not meaningful on the manifold. New mjsActuator.ctrlspec field carries
the signature through the spec and XML round-trip.

Actuator sensors (actuatorpos/vel/frc) now report one value per force
output; dim = 3 on an SO3 actuator.

As the first actuator with nu != nactuator, this commit also makes the
viewers multi-input aware: the control sliders in simulate and studio,
which indexed per-actuator arrays by control index (out of bounds on
this model class), are generated per control and labeled with the
actuator name plus an input suffix ("orient/qw"), via the new
introspection helper mj_actuatorInputName -- the single source of truth
for input names, extended by each new multi-input type (quaternion
components are w-first: qw, qx, qy, qz). Slider ranges now honor a
defined ctrlrange even when ctrllimited is false: range is the UI hint,
limited is the clamp -- wrapped and expmap setpoints are unbounded but
still want finite sliders, while quat components are truly bounded.

The rotational demo model is orientation.xml under
test/engine/testdata/actuation/, upgraded to a three-way contrast:
per-axis wrapped servos vs an expmap-commanded vs a quat-commanded
orientation actuator, on identical checker-textured boxes. It is loaded
by the mixed-axis contrast and input-name tests, and doubles as the
viewer test model (slider groups of 3 independent, 3 grouped, 4 grouped).

PiperOrigin-RevId: 951607063
Change-Id: If235dba8e2f2ca72672e7c62531a27e967c6a373

unity/Runtime/Bindings/MjBindings.cs

@@ -265,6 +265,7 @@ public enum mjtTrn : int{
   mjTRN_TENDON = 3,
   mjTRN_SITE = 4,
   mjTRN_BODY = 5,
+  mjTRN_SO3 = 6,
   mjTRN_UNDEFINED = 1000,
 }
 public enum mjtDyn : int{
@@ -281,14 +282,20 @@ public enum mjtGain : int{
   mjGAIN_AFFINE = 1,
   mjGAIN_MUSCLE = 2,
   mjGAIN_DCMOTOR = 3,
-  mjGAIN_USER = 4,
+  mjGAIN_SO3 = 4,
+  mjGAIN_USER = 5,
 }
 public enum mjtBias : int{
   mjBIAS_NONE = 0,
   mjBIAS_AFFINE = 1,
   mjBIAS_MUSCLE = 2,
   mjBIAS_DCMOTOR = 3,
-  mjBIAS_USER = 4,
+  mjBIAS_SO3 = 4,
+  mjBIAS_USER = 5,
+}
+public enum mjtCtrlChart : int{
+  mjCHART_EXPMAP = 1,
+  mjCHART_QUAT = 2,
 }
 public enum mjtObj : int{
   mjOBJ_UNKNOWN = 0,
@@ -1497,6 +1504,7 @@ public unsafe struct mjModel_ {
   public int* actuator_biastype;
   public int* actuator_ctrladr;
   public int* actuator_ctrlnum;
+  public int* actuator_ctrlspec;
   public int* actuator_outadr;
   public int* actuator_outnum;
   public int* actuator_actadr;
@@ -1518,11 +1526,11 @@ public unsafe struct mjModel_ {
   public int* actuator_group;
   public double* actuator_user;
   public int* actuator_plugin;
+  public byte* actuator_forcelimited;
+  public double* actuator_forcerange;
   public byte* actuator_ctrllimited;
   public double* actuator_ctrlrange;
   public double* actuator_gear;
-  public byte* actuator_forcelimited;
-  public double* actuator_forcerange;
   public double* actuator_acc0;
   public double* actuator_length0;
   public double* actuator_lengthrange;
@@ -6809,6 +6817,9 @@ public static unsafe extern mjData_* mj_copyData(mjData_* dest, mjModel_* m, mjD
 [DllImport("mujoco", CallingConvention = CallingConvention.Cdecl)]
 public static unsafe extern mjData_* mjv_copyData(mjData_* dest, mjModel_* m, mjData_* src);
 
+[DllImport("mujoco", CallingConvention = CallingConvention.Cdecl)]
+public static unsafe extern void mj_resetCtrl(mjModel_* m, mjData_* d);
+
 [DllImport("mujoco", CallingConvention = CallingConvention.Cdecl)]
 public static unsafe extern void mj_resetData(mjModel_* m, mjData_* d);
 
@@ -7085,6 +7096,9 @@ public static unsafe extern int mj_name2id(mjModel_* m, int type, [MarshalAs(Unm
 [DllImport("mujoco", CallingConvention = CallingConvention.Cdecl)]
 public static unsafe extern IntPtr mj_id2name(mjModel_* m, int type, int id);
 
+[DllImport("mujoco", CallingConvention = CallingConvention.Cdecl)]
+public static unsafe extern IntPtr mj_actuatorInputName(mjModel_* m, int id, int input);
+
 [DllImport("mujoco", CallingConvention = CallingConvention.Cdecl)]
 public static unsafe extern void mj_fullM(mjModel_* m, mjData_* d, double* dst);