Ping360 segment offset/sector direction

## Summary
Would be useful to have a segment offset option to set the primary direction of a sector scan.

## Additional information
Would enable scanning just a particular side, or behind, instead of the current options which only allow full 360 scanning or in front.

Likely most intuitive to set the sector middle angle (rather than starting/end angle), because then you set the direction angle based on what you want to point at, and set the sector width (currently called 'Sector Angle (degrees)') based on how much you want to show.

No files, tests, or entry points are named. Start by locating the existing Ping360 sector-scan controls and scan handling, then determine how a sector middle angle and width could support scanning to either side or behind; done means arbitrary sector direction is configurable instead of only full 360-degree or forward scans.

Written by the indexing model from the issue text.