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

python/mujoco/introspect/functions.py

@@ -11035,6 +11035,69 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
          ),
          doc='Set actuator to orientation servo.',
      )),
+    ('mjs_setToPID',
+     FunctionDecl(
+         name='mjs_setToPID',
+         return_type=PointerType(
+             inner_type=ValueType(name='char', is_const=True),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='actuator',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsActuator'),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='kp',
+                 type=ValueType(name='double'),
+             ),
+             FunctionParameterDecl(
+                 name='kv',
+                 type=ArrayType(
+                     inner_type=ValueType(name='double'),
+                     extents=(1,),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='dampratio',
+                 type=ArrayType(
+                     inner_type=ValueType(name='double'),
+                     extents=(1,),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='ki',
+                 type=ArrayType(
+                     inner_type=ValueType(name='double'),
+                     extents=(1,),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='imax',
+                 type=ArrayType(
+                     inner_type=ValueType(name='double'),
+                     extents=(1,),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='slewmax',
+                 type=ArrayType(
+                     inner_type=ValueType(name='double'),
+                     extents=(1,),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='inheritrange',
+                 type=ValueType(name='double'),
+             ),
+             FunctionParameterDecl(
+                 name='ctrlspec',
+                 type=ValueType(name='int'),
+             ),
+         ),
+         doc='Set actuator to PID controller.',
+     )),
     ('mjs_setToDamper',
      FunctionDecl(
          name='mjs_setToDamper',