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/enums.py

@@ -266,7 +266,8 @@ ENUMS: Mapping[str, EnumDecl] = dict([
              ('mjDYN_FILTEREXACT', 3),
              ('mjDYN_MUSCLE', 4),
              ('mjDYN_DCMOTOR', 5),
-             ('mjDYN_USER', 6),
+             ('mjDYN_PID', 6),
+             ('mjDYN_USER', 7),
          ]),
      )),
     ('mjtGain',
@@ -279,7 +280,8 @@ ENUMS: Mapping[str, EnumDecl] = dict([
              ('mjGAIN_MUSCLE', 2),
              ('mjGAIN_DCMOTOR', 3),
              ('mjGAIN_SO3', 4),
-             ('mjGAIN_USER', 5),
+             ('mjGAIN_PID', 5),
+             ('mjGAIN_USER', 6),
          ]),
      )),
     ('mjtBias',
@@ -304,6 +306,16 @@ ENUMS: Mapping[str, EnumDecl] = dict([
              ('mjCHART_QUAT', 2),
          ]),
      )),
+    ('mjtCtrlInput',
+     EnumDecl(
+         name='mjtCtrlInput',
+         declname='enum mjtCtrlInput',
+         values=dict([
+             ('mjINPUT_POS', 1),
+             ('mjINPUT_VEL', 2),
+             ('mjINPUT_FF', 4),
+         ]),
+     )),
     ('mjtObj',
      EnumDecl(
          name='mjtObj',