Extend Documentation for MainDirection and Velocity

Description

Add clear documentation for the MainDirection and Velocity methods. Both return float values:

• MainDirection: distance from the entry point in mm (calculated post-surgery for each frame)
• Velocity: movement speed between frames in mm/s

Acceptance Criteria

• Purpose of each method is briefly explained
• Parameters and return values are documented (units: mm, mm/s)
• Edge cases (no movement, reverse motion, jitter) are noted
• Example usage or expected values are included
• Documentation is visible in code or markdown