Fix Array.union keeping duplicates when one input is empty - #9616

Array.union and Array.unionWith document that they remove duplicates, and Chunk.union, which delegates to Array.union, shows duplicates inside a single input being removed in its JSDoc example. The fast path for an empty operand returned the other operand untouched, so whether duplicates were removed depended on whether the other side happened to be empty:

Array.union([1, 1, 2], [2]) // [1, 2]
Array.union([1, 1, 2], [])  // [1, 1, 2]
Chunk.union(Chunk.make(1, 1, 2), Chunk.empty()) // Chunk(1, 1, 2)

union and unionWith now always deduplicate the concatenated inputs, so the result is a set regardless of which operand is empty. Both dedupe helpers keep the first element, so the NonEmptyArray overloads still hold at runtime. Regression cases cover both operand orders for union, unionWith and Chunk.union.

The unionWith equivalence is now typed as (A | B, A | B) => boolean. Deduplication compares elements from the same side and in either order, so the previous (A, B) signature was unsound whenever both inputs were nonempty. Type tests reject a callback that only accepts (A, B).

Closes EFF-2579