Fix touchdown detection logic in SwingTrajectoryController

[amlansahoo07]

The check_touch_down_condition() method in SwingTrajectoryController had incorrect logic that detected lift-off events instead of touchdown events. This caused step frequency optimization to fail with slow gaits, particularly crawl patterns at 0.5 Hz step frequency.

Implemented a robust three-stage touchdown detection algorithm:

• Fixed core logic to detect touchdown instead of lift-off
• Added proper state management with automatic reset
• Implemented stability delay to prevent premature optimization triggers

This fix enables proper step frequency optimization for crawl gaits and improves the reliability of gait adaptation across all locomotion patterns.

[giulioturrisi]

Thanks Amlan. I'm planning to review this PR this week.