Alignment tools for labels don't work inside of participant/lane

Describe the Bug

Alignment helpers (snap lines) for external labels show outside of participant, but not within participant / lane:

Steps to Reproduce

• Align external label with Gateway/Event outside of participant
  • See that label target is "peer" used for horizontal center alignment
• Align external label with Gateway/Event inside of participant
  • See that label target is not accounted as "peer", and no assistance is given

Can be reproduced on demo.bpmn.io.

Expected Behavior

Same behavior, independent of the modeling context.

Environment

• Browser: Chrome 148
• OS: Any (Linux, Ubuntu 24)
• Library version: v18.9.1 / demo.bpmn.io