Clarify CAN bridge dependency

[s2015-turtle]

Summary

• clarify that the core driver communicates through can_msgs/msg/Frame topics
• document that compatible CAN bridges can be used instead of ros2_socketcan
• keep the direct ros2_socketcan dependency confined to robstride_examples
• make plugin metadata, timeout logging, and the changelog backend-neutral

Why

The package manifests already had the intended dependency boundary, but the README and core-facing text made the Hardware Component appear specific to ros2_socketcan.

User impact

Users can install robstride_ros2_control with their preferred can_msgs/msg/Frame bridge. The example and aggregate packages continue to provide the existing ros2_socketcan launch path.

Validation

• confirmed ros2_socketcan is a direct dependency only in robstride_examples/package.xml
• validated package and plugin XML with xmllint
• ran git diff --check
• no local build was run because this is a documentation, metadata, and log-message-only change

Closes #3