Keep exclusions local when printing list type unions - #15926

Assisted-by: Codex:GPT-6

add_to_list_normalize/4 can copy exclusions from an existing union alternative onto an incoming alternative that has no exclusions. This changes the meaning of the printed type. In the regression below, the input permits [:b], but every printed alternative excludes it.

When delete_elem/3 finds no matching exclusion, the current code recurses with the existing alternative’s n instead of the incoming alternative’s []. Insert {list, last, []} directly into the accumulator instead. Since n is necessarily nonempty in this clause, the previous recursive call immediately reached the catch-all clause. Direct insertion preserves that behavior while keeping each alternative’s exclusions separate.

The regression uses bare_intersection/2 to retain the structure that needs normalization during printing. It covers both leaking exclusions and incorrectly absorbing the incoming alternative into a later branch, which would lose [:d].