Fix themeable._omit inheritance

[has2k1]

_omit was a class attribute consulted by themeable.properties to strip keys that mpl.set could not accept on the artist. Because the themeable hierarchy is inverted — general themeables are Python subclasses of specifics — Python's MRO attribute lookup made a general themeable (rect, axis_text, line) silently inherit _omit from its first specific base. That filter, calibrated for one specific's artist, was then applied to every artist the general themeable composed via super() chaining.

The bug was masked for users by the merge cascade in Themeables.update, but visible under direct themeable.apply (tests, extensions) and latent for any future reader of instance.properties outside apply_*. By happenstance, it was also masked by the default values used to create themes: e.g. when defining theme_gray, if you change the rect to have fill="pink" it would not apply!, but with "white" it went unnoticed.

Replace _omit with themeable._get_properties(omit=...) called inside each apply_* method. The filter is local to the apply call, tied to the mpl_artist.set(**) it feeds, and not subject to MRO attribute leakage. Three previously-defensive
with suppress(KeyError): del props[KEY] blocks in the title themeables (axis_title_x ha, axis_title_y va, plot_title ha) were also unconditional drops that fold cleanly into the helper call.

Composition fix: when the bug was masked, the composition's outer plot_background was incidentally transparent. With the fix it correctly carries rect.fill, exposing a latent layering issue — at zorder=-1000 the outer covered the per-plot tinted backgrounds at the same zorder. Move the composition outer to zorder=-2000 so per-plot backgrounds layer above it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.81%. Comparing base (781f680) to head (b105dec).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1056      +/-   ##
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- Coverage   86.84%   86.81%   -0.03%     
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  Files         203      203              
  Lines       13788    13759      -29     
  Branches     1689     1689              
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- Hits        11974    11945      -29     
  Misses       1265     1265              
  Partials      549      549              

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