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

doc/APIreference/APItypes.rst

@@ -357,6 +357,16 @@ Orientation input charts of so3 actuators. These values are used in ``m->actuato
 .. mujoco-include:: mjtCtrlChart
 
 
+.. _mjtCtrlInput:
+
+mjtCtrlInput
+~~~~~~~~~~~~
+
+Input bitflags of servo-family (pd, dcmotor) actuators. These values are used in ``m->actuator_ctrlspec``.
+
+.. mujoco-include:: mjtCtrlInput
+
+
 .. _mjtObj:
 
 mjtObj

doc/APIreference/functions.rst

@@ -5315,6 +5315,15 @@ Set actuator to velocity servo; return error if any.
 
 Set actuator to orientation servo.
 
+.. _mjs_setToPID:
+
+`mjs_setToPID <#mjs_setToPID>`__
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_setToPID
+
+Set actuator to PID controller.
+
 .. _mjs_setToDamper:
 
 `mjs_setToDamper <#mjs_setToDamper>`__