Add Tree::wakeUpSignal() getter

[tonynajjar]

Problem

Tree::sleep() uses WakeUpSignal::waitFor(), which internally calls std::condition_variable::wait_for() — this always measures time against steady_clock (wall time). When using a simulated clock that runs faster or slower than real time (e.g., ROS 2's /clock topic with use_sim_time), the tree tick rate doesn't scale with the simulation. At 10x sim speed, a 100ms loop timeout still blocks 100ms real time, so the tree ticks 10x too slowly relative to the simulated world.

C++ provides no mechanism to make condition_variable wait on a custom clock, so any solution requires polling.

Solution

Add Tree::wakeUpSignal() — a simple getter that returns the tree's shared WakeUpSignal. This allows frameworks to implement their own clock-aware sleep logic externally while still being able to check for wake-up signals (preemption).

For example, in ROS 2 (Nav2), the between-tick sleep loop can poll the ROS clock for the deadline while doing short waitFor(1ms) calls to stay responsive to preemption:

auto wake_up = tree->wakeUpSignal();
while (ros_clock->now() < deadline) {
  if (wake_up->waitFor(std::chrono::milliseconds(1)))
    return true;  // woken up early
}