TimeSource: apply use_sim_time from a post-set callback, drop /parameter_events subscription

[jplapp]

Why

Every node's TimeSource subscribes to /parameter_events only to notice a change of its own use_sim_time (upstream TODO ros2#609). With N nodes, each declare_parameter of each node is delivered to all N nodes and dropped by N-1 of them. On an AMR (234 nodes) that is about 10k events to about 210 subscribers during startup.

Change

use_sim_time is applied in a local post-set-parameters callback. The subscription and the on_parameter_event handler are removed.

Measured on amr47 (stack restart, container start until nav2 "Managed nodes are active")

	time	/parameter_events subscribers
before (3 boots)	70.6 / 71.2 / 72.7 s	210
after (2 boots)	59.6 / 62.0 s	24

All 234 nodes came up, with the same died/traceback counts in the launch log as before.

Behaviour notes

• The value given at startup (launch argument / yaml) is handled exactly as before.
• When use_sim_time is switched on at runtime, the /clock subscription is created without the QoS override parameters, because parameters cannot be declared from within a parameter callback. (This is the reason upstream went through the topic.) Tested on an isolated domain with static_transform_publisher: ros2 param set ... use_sim_time true/false/true/false/true all succeed and the /clock subscription appears and disappears.
• Deleting use_sim_time is no longer tracked (parameter_state_ = UNSET); the parameter is declared by the TimeSource itself.
• Lock order, for review: the new callback takes node_base_lock_ while the parameter mutex is held; attachNode takes them the other way round, but registers the callback as its last step.
• Nodes that read other nodes' parameter events keep working, since the publishers are unchanged: foxglove_bridge, SmacPlannerHybrid (global costmap resolution), the rosbag recorder.

Not run: the rclcpp unit tests (test_time_source). Built with BUILD_TESTING=OFF on the robot and on the dev machine. The version tested on the robot still contained the unused on_parameter_event; its removal was compile-checked only.

🤖 Generated with Claude Code

https://claude.ai/code/session_011FfCzN8XKNXLTuCXBWJcK5