Deprecate unused structures: `LineSeg`, `Regression`

During recent code review, I noticed that the following structures are no longer used anywhere in the codebase:

* `LineSeg`
* `Regression`

### Proposal

* Mark both structures as **deprecated**.
* Remove them in a future cleanup PR.

### Benefits

* Reduces codebase clutter.
* Avoids confusion for contributors who might assume these structures are still active parts of the system.




