fix(topology): parent_faces initialization silently discards SetSize

[ahojukka5]

MeshTopology::BuildParentFaces (build_parent_faces path) does:

parent_faces.SetSize (nfa);
parent_faces = { -1, { -1, -1, -1, -1 } };

The second line looks like a broadcast-initialize of every entry to a default "no parent" sentinel, but Array::operator=(std::initializer_list) (core/array.hpp) is implemented as *this = Array<T>(list); return *this; -- with a single-element list, this constructs a size-1 Array and assigns it, silently REPLACING the array SetSize(nfa) had just allocated. The array's true size after this line is 1, not nfa.

The loop immediately below only assigns parent_faces[i] for faces that go through one of several conditional branches (a face on the coarse level, a newly-created midpoint face, etc.) -- any face index that reaches none of those branches is never explicitly written. Reading such an index via GetParentFaces then returns data at an out-of-bounds offset into a size-1 array -- observed in practice as non-deterministic values in 3 of the 4 returned parent-face slots, varying between runs on identical input.

Fix: explicitly loop and initialize every one of the nfa entries to the intended sentinel before the per-face population loop runs, instead of relying on the broadcast-looking assignment.

[Copilot]

Pull request overview

Fixes a correctness issue in MeshTopology::BuildParentFaces where an initializer-list assignment unintentionally resizes parent_faces to length 1 after SetSize(nfa), leading to out-of-bounds reads via GetParentFaces.

Changes:

• Replace the misleading initializer-list assignment with explicit per-entry initialization to the intended “no parent” sentinel.
• Add an in-code note explaining why initializer-list assignment is unsafe here (due to Array::operator=(std::initializer_list<T>) semantics).

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