Variable mergeDuplicateVerticesConst

mergeDuplicateVertices: Fix = ...

Auto-fix for GEOMETRY_DUPLICATE_VERTICES — coalesces vertex slots that are byte-identical across every populated attribute (positions, normals, UVs, colors) and remaps indices / edgeIndices to point at the canonical slot.

Vertex splitting for genuine seams / crease edges produces different normals, UVs, or colors on the same position, so those slots fingerprint differently and stay separate. Only truly redundant slots coalesce.

Compacted arrays are sized to the canonical-slot count, so this fix also implicitly subsumes compactUnusedVertices for any vertices that became unused as a side-effect.

Idempotent: returns {fixed: false} when every vertex slot is already unique under the (positions + normals + UVs) key.