DART upgraded from 6.13 to 6.16.6 in Linux Gazebo Jetty (gz-physics9)

The Gazebo robotics simulator team has issued a critical, out-of-cycle update for its stable Gazebo Jetty distribution, upgrading the underlying DART physics engine from version 6.13 to 6.16.6. This move is described as "exceptional" for a stable release but was deemed necessary to resolve a long-standing, high-impact bug that has plagued users for over three years. The update is currently available for Ubuntu users of the Gazebo Jetty packages (specifically gz-physics9) and is planned for a wider rollout to other stable Gazebo distributions pending regression testing. The core driver for this urgent upgrade is the fix for issue gz-sim#1684, where joints with position limits would become permanently unresponsive upon hitting their boundaries.

The key technical fix is found in DART version 6.16.0, which introduced a patch (dartsim/dart#2086) allowing "servo joints to recover from position limits." Previously, a joint reaching its limit could freeze indefinitely, ignoring new velocity commands. In force-control mode, this bug caused extreme PID integral windup, resulting in recovery delays exceeding 30 seconds—a catastrophic failure for precise robotic simulation. The Gazebo team has conducted extensive automated testing against the gz-physics and gz-sim test suites (tracked in gz-physics#816) and reports no major regressions. The update is delivered via the `libgz-physics-dartsim-plugin.so.9.1.0` plugin, maintaining ABI/API stability. For researchers and developers, this fix dramatically improves the reliability of simulations involving complex, multi-jointed robots and closed-loop control systems, making Gazebo Jetty a more robust platform for development and testing.