Fix: Core logic improvements and bug fixes

[JorenB]

This commit addresses several potential issues and improves the robustness of the CDT simulation code.

Key changes include:

1. RNG Seeding: Standardized random number generator seeding for Universe::rng, ensuring it's seeded from main like Simulation::rng. Addresses a TODO comment.

2. verticesFour Management:

   • insertVertex: Correctly updates the verticesFour bag for the newly inserted vertex and its relevant neighbors by evaluating isFourVertex.
   • removeVertex: Ensures verticesFour is updated for neighbors affected by a vertex removal.
   • flipLink: Systematically updates verticesFour for all four vertices involved in an edge flip using isFourVertex. The setVertices calls within flipLink were also updated to reflect a standard robust edge flip procedure.

3. Universe::check() Enhancement: The verticesFour verification in Universe::check() now iterates over all active vertices in the simulation (using Vertex::items()) and asserts that their status (per isFourVertex) matches their presence in the verticesFour bag.

4. Universe::vertices List: Universe::updateVertexData now populates Universe::vertices with all unique active vertices. This ensures that Universe::exportGeometry exports the complete geometry.

5. Robust Geometry I/O:

   • Universe::importGeometry now returns a boolean status indicating success or failure instead of asserting on errors.
   • It handles file open errors, read errors, and count mismatches gracefully, printing informative messages.
   • main.cpp now checks this return status and proceeds accordingly, allowing for fallback to new universe creation if import fails.

The trianglesFlip logic within the move functions was largely preserved to limit the scope of this set of changes, though the corrected setVertices in flipLink should contribute to its correctness. Further investigation into trianglesFlip invariants might be beneficial in the future.