Suppress sharing warnings when no sharing is possible

[kinyatoride]

Suppress sharing warnings when no sharing is possible

Problem

UltraPlotWarnings about axis sharing fired in cases where the user did not request sharing or where sharing was not possible at all:

• uplt.subplots(proj='polar', ncols=2, nrows=3, share=0) — the user explicitly disabled sharing, but construction and every subsequent draw emitted Skipping incompatible y-axis sharing for PolarAxes and PolarAxes: different axis families. and Tick label sharing not implemented for <class 'ultraplot.axes.polar.PolarAxes'> subplots.
• uplt.subplot(proj='polar') / uplt.subplots(proj='polar') — a single polar axis has nothing to share with, but Tick label sharing not implemented for <PolarAxes> subplots. still fired four times per draw.

Fix

Two unrelated paths needed gating:

• Axes._apply_auto_share (ultraplot/axes/base.py) called Figure._partition_share_axes for x and y unconditionally. Wrapped the external-share blocks in if self.figure._sharex > 0: / if self.figure._sharey > 0:, mirroring the existing panel-share guards just above.

• Figure._share_ticklabels (ultraplot/figure.py) ran _compute_baseline_tick_state (which emits the Tick label sharing not implemented warning for unsupported axes types) before checking whether the group could actually share. Skip the group up front when it has fewer than two axes or when no axes have effective share level >= 3.

Explicit share='all' (or any level >= 1) on multiple polar / mixed cart-polar grids still warns as before — that case is informative.

Tests

Added regression tests in ultraplot/tests/test_figure.py:

• test_share_zero_polar_emits_no_warnings — share=0 on an all-polar grid.
• test_share_zero_mixed_cartesian_polar_emits_no_warnings — share=0 on a mixed cart/polar grid.
• test_share_default_single_polar_emits_no_warnings — default share on a single-axis polar figure via uplt.subplots.
• test_share_default_single_polar_subplot_singular_emits_no_warnings — same via uplt.subplot (singular).

Existing test_explicit_share_warns_for_mixed_cartesian_polar continues to pass, confirming the explicit-share warning path is preserved. All 23 share-related tests pass; broader test_figure.py / test_subplots.py / test_projections.py (139 tests) also pass.