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

unity/Runtime/Bindings/MjBindings.cs

@@ -276,7 +276,8 @@ public enum mjtDyn : int{
   mjDYN_FILTEREXACT = 3,
   mjDYN_MUSCLE = 4,
   mjDYN_DCMOTOR = 5,
-  mjDYN_USER = 6,
+  mjDYN_PID = 6,
+  mjDYN_USER = 7,
 }
 public enum mjtGain : int{
   mjGAIN_FIXED = 0,
@@ -284,7 +285,8 @@ public enum mjtGain : int{
   mjGAIN_MUSCLE = 2,
   mjGAIN_DCMOTOR = 3,
   mjGAIN_SO3 = 4,
-  mjGAIN_USER = 5,
+  mjGAIN_PID = 5,
+  mjGAIN_USER = 6,
 }
 public enum mjtBias : int{
   mjBIAS_NONE = 0,
@@ -298,6 +300,11 @@ public enum mjtCtrlChart : int{
   mjCHART_EXPMAP = 1,
   mjCHART_QUAT = 2,
 }
+public enum mjtCtrlInput : int{
+  mjINPUT_POS = 1,
+  mjINPUT_VEL = 2,
+  mjINPUT_FF = 4,
+}
 public enum mjtObj : int{
   mjOBJ_UNKNOWN = 0,
   mjOBJ_BODY = 1,