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

include/mujoco/mujoco.h

@@ -1760,6 +1760,11 @@ MJAPI const char* mjs_setToVelocity(mjsActuator* actuator, double kv);
 MJAPI const char* mjs_setToOrientation(mjsActuator* actuator, double kp, double kv[1],
                                        double dampratio[1], int ctrlspec);
 
+// Set actuator to PID controller.
+MJAPI const char* mjs_setToPID(mjsActuator* actuator, double kp, double kv[1], double dampratio[1],
+                               double ki[1], double imax[1], double slewmax[1], double inheritrange,
+                               int ctrlspec);
+
 // Set actuator to activate damper; return error if any.
 MJAPI const char* mjs_setToDamper(mjsActuator* actuator, double kv);