CI: Test the wheel without matplotlib

[bashtage]

Branch a90-ci-no-matplotlib, one commit; #960, which it was built on, is merged, so the diff is only CI files (ci/azure/azure_template_posix.yml, ci/azure/install-posix.sh).

The arch-wheels builds test the installed wheel without matplotlib. arch's own CI
always installs it, so three new test modules that imported matplotlib at module
level passed every check here and failed in every arch-wheels job (#960). This
job catches that in CI:

• New Azure Linux leg python_313_no_matplotlib_wheel: Python 3.13 with
  NO_MATPLOTLIB: 'true' and test.wheel: true. It builds the wheel, installs it
  in site-packages and runs arch.test() from a directory outside the source
  tree, as the arch-wheels builds do. arch.test() exits with pytest's status, so
  a collection error fails the job.
• install-posix.sh: with NO_MATPLOTLIB, it leaves out matplotlib and
  seaborn, which needs it. After the install, it fails if matplotlib can be
  imported anyway, for example because another package pulled it in, so the job
  cannot silently test with matplotlib.

The comparison is case-insensitive, because Azure can pass a YAML true as
"True". The other legs are unchanged. Checked locally: the script's syntax
(bash -n), the YAML (11 Linux legs), and the absence check with and without
matplotlib.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.95%. Comparing base (d2c90b4) to head (b2c5c1a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #961   +/-   ##
=======================================
  Coverage   98.95%   98.95%           
=======================================
  Files         136      136           
  Lines       30043    30043           
  Branches     2331     2331           
=======================================
  Hits        29729    29729           
  Misses        256      256           
  Partials       58       58           

Flag	Coverage Δ
adder	98.89% <ø> (ø)
subtractor	98.89% <ø> (ø)

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