fix(set): setUnion returns wrong element order when sets overlap

[JSap0914]

Problem

math.setUnion() produces incorrect output whenever the two input sets share elements — the intersection is placed at the end instead of its sorted position.

math.setUnion([1, 2, 3, 4], [3, 4, 5, 6])
// before: [1, 2, 5, 6, 3, 4]  ← wrong (intersection at end)
// after:  [1, 2, 3, 4, 5, 6]  ← correct (matches JSDoc example)

Root Cause

The previous implementation:

return concat(setSymDifference(b1, b2), setIntersect(b1, b2))

setSymDifference returns [a-only, b-only], so the result is [a-only, b-only, a∩b] — the shared elements land last.

Fix

Replace with three explicit pieces that are each sorted by their respective helpers:

return concat(concat(setDifference(b1, b2), setIntersect(b1, b2)), setDifference(b2, b1))

This gives the correct sorted union: a-only ++ common ++ b-only.

Changes

• src/function/set/setUnion.js: swap setSymDifference dependency for setDifference; fix the formula.
• test/unit-tests/function/set/setUnion.test.js: add overlapping-set assertions (were RED); correct the multiset expected value (was documenting wrong output).

Closes #3674

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