Add <damper> actuator shortcut and related mjGAIN_AFFINE.

**Rationale**

A general `mjGAIN_AFFINE` actuator adds the term `force = (c + kp * length + kv * velocity) * ctrl`. The damper shortcut restricts it to `kv * velocity * ctrl` and sets the `ctrllimited` attribute to true.

**API**

```xml
<actuator>
  <damper ctrlrange="0 1" kv="1"/>
</actuator>
```

* Required attributes: `ctrlrange` (>=0)
* Optional attributes: `kv` (>=0)

PiperOrigin-RevId: 455366866
Change-Id: I88773abbeccabb6442f215c9fc05e205df63f692

src/engine/engine_derivative.c

@@ -735,33 +735,41 @@ static void mjd_actuator_vel(const mjModel* m, mjData* d, mjtNum* DfDv) {
 
   // process actuators
   for (int i=0; i<m->nu; i++) {
+    mjtNum bias_vel = 0, gain_vel = 0;
+
     // affine bias
     if (m->actuator_biastype[i]==mjBIAS_AFFINE) {
       // extract bias info: prm = [const, kp, kv]
-      mjtNum* prm = m->actuator_biasprm + mjNBIAS*i;
+      bias_vel = (m->actuator_biasprm + mjNBIAS*i)[2];
+    }
 
-      // add
-      mjtNum B = prm[2];
-      addJTBJ(DfDv, d->actuator_moment+i*nv, &B, 1, nv);
+    // affine gain
+    if (m->actuator_gaintype[i]==mjGAIN_AFFINE) {
+      // extract bias info: prm = [const, kp, kv]
+      gain_vel = (m->actuator_gainprm + mjNGAIN*i)[2];
     }
 
     // muscle gain
     else if (m->actuator_gaintype[i]==mjGAIN_MUSCLE) {
-      mjtNum B = mjd_muscleGain_vel(d->actuator_length[i],
+      gain_vel = mjd_muscleGain_vel(d->actuator_length[i],
                                     d->actuator_velocity[i],
                                     m->actuator_lengthrange+2*i,
                                     m->actuator_acc0[i],
                                     m->actuator_gainprm + mjNGAIN*i);
+    }
 
-      // force = gain .* [ctrl/act]
+    // force = gain .* [ctrl/act]
+    if (gain_vel!=0) {
       if (m->actuator_dyntype[i]==mjDYN_NONE) {
-        B *= d->ctrl[i];
+        bias_vel += gain_vel * d->ctrl[i];
       } else {
-        B *= d->act[i-(m->nu - m->na)];
+        bias_vel += gain_vel * d->act[i-(m->nu - m->na)];
       }
+    }
 
-      // add
-      addJTBJ(DfDv, d->actuator_moment+i*nv, &B, 1, nv);
+    // add
+    if (bias_vel!=0) {
+      addJTBJ(DfDv, d->actuator_moment+i*nv, &bias_vel, 1, nv);
     }
   }
 }

src/engine/engine_forward.c

@@ -197,6 +197,10 @@ void mj_fwdActuation(const mjModel* m, mjData* d) {
       gain = prm[0];
       break;
 
+    case mjGAIN_AFFINE:             // affine: prm = [const, kp, kv]
+      gain = prm[0] + prm[1]*d->actuator_length[i] + prm[2]*d->actuator_velocity[i];
+      break;
+
     case mjGAIN_MUSCLE:             // muscle gain
       gain = mju_muscleGain(d->actuator_length[i],
                             d->actuator_velocity[i],