Keep void-node constructor in homogeneous gene during validation

[chrxh]

Problem

In the genome editor, genes reachable only through a homogeneous gene's void nodes were shown as unreachable (e.g. the whole graph collapsing to just gene 0 + gene 4), even though the engine keeps and builds them.

Root cause: the editor runs DescValidationService::validateAndCorrect when a genome is set (GenomeTabWidget), and that function stripped the constructor from every void node:

if (nodeType == CellType_Void) {
    node._constructor = std::nullopt;
}

But in a homogeneous gene the effective cell type is taken from the first node, so a void node is actually expressed as that type and legitimately keeps its constructor — exactly as ConstructorProcessor::createConstructionData (uses result.node->constructor) and MutationProcessor::applyMutations (MutationProcessor.cuh:823 only clears constructorAvailable when !homogeneousCellType) do. So the display dropped live references the engine relies on.

Reproduction (References.sim, one timestep)

Genome	reachable genes	gene 4 refs
Engine (raw)	12	[0,5]
After validateAndCorrect (before fix)	2	[]
After validateAndCorrect (after fix)	12	[0,5]

The reachable=2 case exactly matches the reported "only gene 0 and gene 4 reachable" symptom.

Fix

Strip the constructor from a void node only in non-homogeneous genes, mirroring the engine.

Tests

• New regression test genome_keepsConstructorOnVoidCellInHomogeneousGene.
• Existing genome_removesConstructorFromVoidCell (non-homogeneous) still passes.
• All 171 EngineInterfaceTests pass.

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