Require explicit motor gains

[s2015-turtle]

Summary

• require kp and kd on every motor-profile Xacro macro
• preserve the previous example values by passing them explicitly in repository fixtures
• add parameterized tests proving that omitted gains fail and explicit gains expand
• document that active gains must be tuned for the robot and are not model defaults

Why

The previous macro defaults could apply active stiffness and damping values without an explicit robot-level decision.

User impact

Existing raw ros2_control joint parameters are unchanged. Users of the profile macros must now provide kp and kd explicitly.

Validation

• git diff --check
• XML validation with xmllint
• Python syntax compilation
• CI will execute the new Xacro pytest on all supported ROS 2 distributions
• no local build was run on this machine

Closes #6