Implement the skipped stacked bar regression test

[cvanelteren]

ultraplot/tests/test_1dplots.py contains a skipped test_bar_stack() placeholder with no implementation yet. This is a good candidate for a small test-focused contribution.

This issue is intentionally scoped for newcomers to the project. It is low complexity, mostly test-only work, and should help someone get familiar with the plotting API and the existing pytest / pytest-mpl test patterns.

Suggested scope:

• replace the skipped test_bar_stack() with a real regression test
• verify the expected stacking behavior for bar plots
• include area stacking only if it fits naturally without expanding the scope too much

Acceptance criteria:

• the skip marker is removed
• the new test passes reliably in CI
• the test docstring or assertions make the expected stacking behavior clear