Add the pid actuator: setpoint inputs, integral action, slew rate limiting.

<pid kp kv|dampratio [ki imax] [slewmax]> is a PID controller with real position and velocity setpoint inputs on a single force output, plus an optional feedforward input. With a zero velocity setpoint it reproduces <position> bit-exactly; the input signature is any subset of [pos, vel, ff], selected with input="..." and recorded as mjtCtrlInput bits in
actuator_ctrlspec; absent setpoint inputs are fixed at zero, so the control vector contains no inert entries.

kp and kv are single-sourced in the affine bias parameters (biasprm[1,2]) with no gainprm mirror: every consumer of the position-servo shape
(dampratio conversion, inheritrange, qDeriv) reads one location, which is what makes the bit-exact <position> parity possible. Controller state uses dyntype 'pid' with slot-gated activations in the order [slew, integral], following the dcmotor slot idiom: slewmax (dynprm[1]) rate limits the effective position setpoint through an activation holding it;
ki (gainprm[0]) integrates the position error -- wrapped on rotational transmissions -- with anti-windup clamping of the integrand at imax (dynprm[0]). Both features require the pos input. Servo input unpacking is shared with the dcmotor controller (unpackServoInputs); per-input ranges are exposed as posrange/velrange/ffrange.

This subsumes the functionality of the mujoco.pid plugin with proper activation state: correct under all integrators, visible to keyframes, act sensors and reset. Migration: kp/ki/kd map to kp/ki/kv, plugin imax is in force units (divide by ki), slewmax carries over; the single ctrl becomes input="pos".

PiperOrigin-RevId: 957588898
Change-Id: Id2786836ca6e76f58e5b5cc8323fc23be0a53784

src/engine/engine_setconst.c

@@ -1138,8 +1138,9 @@ static void set0(mjModel* m, mjData* d) {
     mjtNum* biasprm = m->actuator_biasprm + i*mjNBIAS;
     mjtNum* gainprm = m->actuator_gainprm + i*mjNGAIN;
 
-    // not a position-like actuator: skip
-    if (gainprm[0] != -biasprm[1]) {
+    // not a position-like actuator: skip (PID single-sources kp in biasprm[1])
+    int is_pid = m->actuator_gaintype[i] == mjGAIN_PID;
+    if (!is_pid && gainprm[0] != -biasprm[1]) {
       continue;
     }
 
@@ -1165,7 +1166,8 @@ static void set0(mjModel* m, mjData* d) {
     }
 
     // damping = dampratio * 2 * sqrt(kp * mass)
-    mjtNum damping = biasprm[2] * 2 * mju_sqrt(gainprm[0] * mass);
+    mjtNum kp = is_pid ? -biasprm[1] : gainprm[0];
+    mjtNum damping = biasprm[2] * 2 * mju_sqrt(kp * mass);
 
     // set biasprm[2] to negative damping
     biasprm[2] = -damping;