fix: dedupe boxplot ticks on shared axes

[saschabuehrle]

Bug

Fixes #628 — with shared x-axes, repeated boxplot() calls can leave duplicated tick positions/labels (e.g. 0..9 rendered twice), which causes doubled/bold tick labels.

Fix

• After the native boxplot call, deduplicate managed axis tick positions while preserving order.
• Apply this only when manage_ticks is active, so explicit tick management still behaves as requested.

Tests

• Added regression test: test_boxplot_shared_x_does_not_duplicate_ticks
• Ran:
  • pytest ultraplot/tests/test_statistical_plotting.py -k boxplot_shared_x_does_not_duplicate_ticks

Happy to adjust if you prefer a different location for the dedupe logic.

Greetings, saschabuehrle

[cvanelteren]

Thanks! Closing in favor of #630