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/changelog.rst

@@ -5,6 +5,23 @@ Changelog
 Upcoming version (not yet released)
 -----------------------------------
 
+Actuation
+^^^^^^^^^
+- Added the :ref:`pid<actuator-pid>` actuator: a PID controller with real position and velocity setpoint inputs,
+  optional integral action (:ref:`ki<actuator-pid-ki>`, integrating the position error with
+  :ref:`imax<actuator-pid-imax>` anti-windup), setpoint rate limiting (:ref:`slewmax<actuator-pid-slewmax>`), and an
+  optional feedforward input. This subsumes the functionality of the ``mujoco.pid`` plugin with proper activation
+  state: correct under all integrators and visible to keyframes and sensors. With a zero velocity setpoint it is
+  identical to :ref:`position<actuator-position>`. The input signature is any subset of ``[pos, vel, ff]``, selected
+  by :ref:`input<actuator-pid-input>`; absent setpoint inputs are fixed at zero, so the control vector contains no
+  inert entries.
+
+.. admonition:: Breaking ABI changes
+   :class: caution
+
+   - :ref:`mjsActuator` gained ``velrange`` and ``ffrange`` fields, changing its size and layout. The :ref:`mjtGain`
+     and :ref:`mjtDyn` enums gained ``pid`` members, shifting the values of ``mjGAIN_USER`` and ``mjDYN_USER``.
+
 Engine
 ^^^^^^