Correct complement set symmetric differences

[a3ylf]

complement({1, 2}) ^ {2, 3} currently returns complement({1}), so it still contains 3 even though 3 belongs to both operands. symmetric_difference_update() gives a different wrong result: it leaves 2 excluded although 2 belongs only to the right operand.

Apply symmetric difference to the stored finite sets when exactly one operand is complemented. This follows (~A) ^ B == ~(A ^ B) and fixes both the operator and the mutating method.

Add 40 regression cases covering overlapping, identical, disjoint, and empty inputs; ordinary sets, frozensets, and both complement representations; reversed operands; and applying the update twice to restore the original membership. Expected results come from ordinary set operations on sampled membership, including values outside both finite inputs.

Validation on Linux:

• Before the fix: 19 of the 40 regression cases fail.
• Python 3.10.20: python -m pytest --doctest-modules boltons tests -q — 712 passed.
• Python 3.14: uvx --with tox-uv tox -e py314 -- -q — 712 passed, including package build and doctests.
• git diff --check passes.

Made with Astra on Codex