Lyrical: try new executor

[vooon]

Summary

Try the new Callback Group Events executor (EventsCBGExecutor) introduced in ROS 2 Lyrical Luth (rclcpp >= 30.0.0), and fix the resource/ownership problems that prevented mavros::router::Router and mavros::uas::UAS from working cleanly as composable nodes in a component container.

Replaces / fixes

• Fixes Multiple nodes with the same name created if mavros::uas::UAS is loaded into a component container #1919 / supersedes On the way to fixing #1919 #1920 — UAS component ownership failure. The plugin↔UAS reference cycle (UAS owns plugins, plugins held a shared_ptr to UAS) meant the nodes were never destroyed on component unload and got duplicated on reload. This PR breaks the cycle with a raw, non-owning UAS* in plugins (as in On the way to fixing #1919 #1920's approach, but without weak_ptr).
• Supersedes Fix resource management in UAS node #1961 (resource management in UAS) — same ownership-cycle fix plus clean teardown; component unload now fully destroys the UAS and all its plugin nodes, reload does not duplicate them.
• Supersedes Fix:: Reduce CPU usage by enabling intra-process communication and b… #2089 (reduce CPU usage) — enables use_intra_process_comms(true) for the internal MAVLink bus and co-located subscribers (zero-copy when router/UAS/plugins share a process; DDS fallback otherwise).
• Fix high CPU usage in sys_time plugin #2212 / CPU usage 4x higher in ROS 2 than ROS 1 #2031 (high CPU in sys_time) — considered but not adopted: the WallRate→std::chrono change there is unlikely to address the CPU usage issue.

Changes

1. Runtime-selectable executor — mavros::uas::make_executor() honoring the MAVROS_EXECUTOR_TYPE env var (mt default, events). Used by both mavros_node and the UAS plugin executor; EventsCBGExecutor is gated to Lyrical+ (rclcpp >= 30.0.0) with a MultiThreadedExecutor fallback on older distros. Thread count remains configurable via MAVROS_UAS_EXECUTOR_THREADS.
2. Composable correctness — plugin→UAS ownership cycle broken (raw pointer); dynamically created plugin nodes use use_global_arguments(false) so a component container's process-global -r __node:=<container> remaps no longer rename every plugin node (previously caused topic type conflicts).
3. CPU / resources — use_intra_process_comms(true) on mavros_node options and plugin nodes.
4. Composable launch — test_compose.launch.py: fcu_url/gcs_url as launch arguments (no env-specific URL hardcoded), an executor argument (mt default / events / auto), typed parameter resolution via OpaqueFunction, INFO log level.
5. Docs — mavros/README.md documents MAVROS_EXECUTOR_TYPE, MAVROS_UAS_EXECUTOR_THREADS, and test_compose.launch.py usage.
6. CI — fix the failing semgrep Scan check (github-actions-mutable-action-tag): # nosemgrep on GitHub first-party actions, SHA-pin ros-tooling/setup-ros.

Verification

• ctest 13/13 green on Lyrical (uncrustify, flake8, pep257, xmllint, cppcheck, cpplint, gmock/gtest, pytest).
• test_compose.launch.py loads Router + UAS as composable nodes; ros2 component unload removes all nodes; reload shows no duplication.
• MAVROS_EXECUTOR_TYPE=events runs standalone mavros_node (and executor:=events the composable container) with clean shutdown.

Caveats

• EventsCBGExecutor in a component container is upstream-immature on Lyrical (EventsCBGExecutor shutdown + ComponentManager destructor cause double remove and runtime_error on shutdown ros2/rclcpp#3186 shutdown double-remove, #3123 container support, #3129 container refactor): load_node can intermittently time out and list_nodes isn't graph-discoverable. The composable launch therefore defaults to the reliable component_container_mt; executor:=events is opt-in. Standalone mavros_node with the events executor works well.
• MAVROS_EXECUTOR_TYPE defaults to mt, so existing behavior is unchanged.
• A flaky process-exit segfault seen in the dev container for mavros-uas-test was stale fastdds /dev/shm state (gone after reboot, 0/30 runs; also happens on baseline) — not a regression from this PR.