Fix #639: unskip and harden the vcenter colormap test

[jessiedoe03-cpu]

Fixes #639.

test_colormap_vcenter was skipped with "not sure what this does". The test
body already called pcolor(data, vcenter=...), so the intent was there - but
vcenter was never plumbed through PlotAxes._parse_cmap. It fell through to
the artist and raised:

AttributeError: PolyQuadMesh.set() got an unexpected keyword argument 'vcenter'

so the test could not simply be unskipped; the parameter had to work first.

What this does

• Unskips test_colormap_vcenter and replaces the image comparison with
  deterministic assertions: the centre maps to 0.5 in both discrete and
  continuous mode, and vcenter= matches norm_kw={'vcenter': ...}.
• Accepts vcenter in _parse_cmap alongside vmin/vmax, forwarding it into
  norm_kw, and extends the existing "don't combine these with an explicit
  Normalize instance" guard to cover it.

One decision worth your call

When vcenter is given and no norm is specified, this selects the diverging
normalizer. That seemed the only sensible reading, and it is what the original
skipped test implied - but it is a behaviour choice rather than pure plumbing,
so say if you would rather it were explicit or named differently.

Testing

• test_colormap_vcenter: 1 passed, 1 warning in 1.66s
• Whole file: 1 failed, 32 passed, 1 skipped, 97 warnings in 11.66s
• With ultraplot/axes/plot.py reverted and the new test kept: 1 failed, 1 warning in 1.50s -
  so the test genuinely covers the change rather than passing regardless.

Any remaining failures in that file are pre-existing _tkinter.TclError issues
in my environment (no display); they fail on the base commit too, and which one
trips varies with run order. Please let CI have the final word.

---

This patch was written with AI assistance (the code by Google Antigravity, the
review and this pull request prepared with Claude Code) and was reviewed and
tested before submitting by @jessiedoe03-cpu. Happy to revise or close it if it is not the
direction you want.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ultraplot/axes/plot.py	83.33%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!