feat: add in-memory GenericVisualization3D helper

[baraafanah21]

Adds GenericVisualization3D, an IVisualization3D implementation that
collects vertices, faces and lines in memory instead of writing them to a
file, as a first step towards the centralized python displaying
infrastructure outlined in #5615. The collected data is exposed through
vertices() / faces() / lines() accessors so it can be handed to
external plotting backends.

Part of #5615 (step 1 of 3 in the workplan there — pybind11 bindings and
a matplotlib backend will follow in separate PRs to keep review manageable).

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Approach

• face() / faces() mirror the semantics of ObjVisualization3D exactly
  (bulk vertex insert, two-index faces stored as lines, index offsets), so
  the in-memory data matches what the obj writer would produce for the same
  drawing calls.
• line() stores a self-contained point pair and does not append its end
  points to the vertex collection — appending them is an OBJ/PLY file-format
  artifact rather than intended semantics. Happy to change if you prefer
  strict Obj behavior.
• write(ostream) / write(path) emit a one-line debug summary, documented
  as "not a serialization format"; object(name) is a no-op like in
  PlyVisualization3D.
• No changes to IVisualization3D or any existing implementation.

Testing

• Structural unit tests for every interface method.
• A cross-validation test draws the same surfaces (rectangle plane, full
  cylinder, sectoral cone — same parameters as the existing
  SurfaceView3DBase tests) with both ObjVisualization3D and
  GenericVisualization3D via GeometryView3D::drawSurface, parses the obj
  output, and checks the collected data element-for-element: vertex
  positions, face index lists, line endpoints, and colors.
• Full unit test suite: 261/261 locally.

Open questions for reviewers

1. Naming: the issue calls the class GenericVisualization; I used the
   3D suffix for consistency with ObjVisualization3D/PlyVisualization3D.
   Happy to rename if you prefer.
2. faces() accessor vs virtual overload: the zero-argument faces()
   accessor overloads the three-argument virtual IVisualization3D::faces().
   Can rename to getVertices() / getFaces() / getLines() if preferred.
3. line() vertex injection: flagging the choice above in case
   Obj-identical behavior is preferred.