Function weld_indexed 

pub fn weld_indexed(
    positions: &[f32],
    normals: &[f32],
    uvs: Option<&[f32]>,
    indices: &[u32],
) -> Option<(Vec<f32>, Vec<f32>, Option<Vec<f32>>, Vec<u32>)>

Weld positions/normals (3 floats per vertex, equal length), optional uvs (2 floats per vertex), and remap indices.

Returns Some((positions, normals, uvs, indices)) ONLY when at least two vertices actually merged; uvs is Some iff the input uvs was, always 1:1 with the welded positions. Returns None when nothing changes — a mesh that is already welded / all-crease (a swept solid, an indexed mesher, a flat-shaded cube), OR a malformed input (normals not matching positions, empty, a UV array not 2-per-vertex, or an out-of-range index). In every None case the identity remap would reproduce the input byte-for-byte, so the caller keeps its ORIGINAL buffers and skips the copy: no per-element reallocation on the (common) already-welded path, and a malformed input stays invalid-but-present rather than panicking or being re-associated.

Because the decision is purely “did any key collide”, the funnel stays uniform — no per-geometry-type branching. The weld is idempotent: welding a welded mesh returns None.