Add optional backface/frontface culling to Ray3d::intersect_plane

[Mati18505]

Objective

Currently, Ray3d::intersect_plane always returns intersections on both faces of the plane, including backface hits.
This behavior:

• isn’t documented clearly,
• cannot be changed by the user without duplicating the method.

Fixes #20152

Solution

• Introduced PlaneIntersectionMode enum to control intersection behavior.
• Added a plane_hit_mode: PlaneIntersectionMode parameter to Ray3d::intersect_plane.
• This enables filtering for:
  • FrontFaceOnly
  • BackFaceOnly
  • Both (preserves previous behavior)

Testing

Added unit tests.

---

Showcase

ray.intersect_plane(
    Vec3::Z,
    InfinitePlane3d::new(Vec3::Z),
    PlaneIntersectionMode::FrontFaceOnly
),

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[alice-i-cecile]

To me the big question here is if we solidify the fact that a plane has a "front" (the current approach), or if we should be creating a new Halfspace type. I am currently undecided, but we should debate it a bit.

[IQuick143]

Personally I'm not a huge fan of putting everything everything into one universal method. Though the code diff is actually nicer than what I expected. (ex: All the backface-relevant data is already computed anyway and the method is already full of branches.) So I'm not against merging this.

Given that the user has to supply an argument (whose documentation explains the details), so they won't be surprised by what the method does.