Fix outer legend hiding y-tick labels with sharey='labs' (#694)

[cvanelteren]

Closes #694.

Summary

_effective_share_level (in figure.py) was promoting an axes' effective share level to 3 whenever any adjacent panel had a shared axis — including the hidden filled panel that an outer legend (loc='r') creates. That promotion fired _share_ticklabels, whose border-mask then turned labelleft off on the inner subplot because the inner subplot is not on the figure's left border.

The fix filters the "adjacent panels" clause to ignore panels with _panel_hidden=True or _panel_share=False, matching the same predicate _apply_auto_share already uses (shared(paxs)). With that, sharey='labs' no longer loses its tick labels when an outer legend is added next to a subplot.

Tests

Two new flat tests in ultraplot/tests/test_subplots.py:

• test_outer_legend_keeps_ticklabels_with_label_sharing[labs|lims] — the issue Adding a legend to right of plot can remove y-axis tick labels #694 reproducer.
• test_outer_legend_preserves_share_true_ticklabel_hiding — guard so the fix doesn't accidentally re-enable interior tick labels when sharey=True (level 3).

All 455 pre-existing test_subplots/test_figure/test_axes/test_colorbar tests still pass.

Note for follow-up — proper architectural fix needed

This PR is intentionally minimal. It papers over a deeper duplication: tick-label visibility is currently decided in two parallel systems — cartesian.py:_apply_axis_sharing_for_axis (per-axes) and figure.py:_share_ticklabels (figure-level) — that race during draw. The "panel participates in sharing" concept is encoded with four slightly different filters across _apply_auto_share, the span helpers, the cartesian tick-sharing path, and (until this PR) _effective_share_level. The bug lived in exactly that gap.

A proper follow-up should:

1. Introduce a single _panel_participates_in_sharing(p) predicate used by every site that filters panel lists.
2. Centralise the "promote to level 3" logic in one place rather than three (cartesian.py:505, cartesian.py:820/847, figure.py:_effective_share_level).
3. Pick a single owner of tick-label visibility per draw and remove the overlapping logic from the other path.

This aligns with the existing plan to extract layout/sharing logic out of Figure. I'll open a separate tracking issue for it.

[codecov]

Codecov Report

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