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/specs.cc

@@ -1567,6 +1567,23 @@ PYBIND11_MODULE(_specs, m, pybind11::mod_gil_not_used()) {
       },
       py::arg("kp"), py::arg("kv") = -1, py::arg("dampratio") = -1,
       py::arg("ctrlspec") = 0);
+  mjsActuator.def(
+      "set_to_pid",
+      [](raw::MjsActuator* self, double kp, double kv, double dampratio,
+         double ki, double imax, double slewmax, double inheritrange,
+         int ctrlspec) {
+        std::string err = mjs_setToPID(
+            self, kp, kv == -1 ? nullptr : &kv,
+            dampratio == -1 ? nullptr : &dampratio, ki == -1 ? nullptr : &ki,
+            imax == -1 ? nullptr : &imax, slewmax == -1 ? nullptr : &slewmax,
+            inheritrange, ctrlspec);
+        if (!err.empty()) {
+          throw pybind11::value_error(err);
+        }
+      },
+      py::arg("kp"), py::arg("kv") = -1, py::arg("dampratio") = -1,
+      py::arg("ki") = -1, py::arg("imax") = -1, py::arg("slewmax") = -1,
+      py::arg("inheritrange") = 0, py::arg("ctrlspec") = 0);
   mjsActuator.def(
       "set_to_damper",
       [](raw::MjsActuator* self, double kv) {