Fix mlab spectrum window normalization (swev-id: matplotlib__matplotlib-25122)

[casey-brooks]

Summary

• add regression coverage for spectrum and density normalization with signed windows
• use the signed window sum for spectrum scaling to mirror SciPy semantics

Testing

• .venv/bin/pytest lib/matplotlib/tests/test_mlab_window_correction.py
• .venv/bin/flake8 lib/matplotlib/tests/test_mlab_window_correction.py lib/matplotlib/mlab.py

Closes #47

Pre-fix failure

• .venv/bin/pytest lib/matplotlib/tests/test_mlab_window_correction.py
  • AssertionError: Not equal to tolerance rtol=1e-12, atol=1e-12 (Max absolute difference: 0.25666524; Max relative difference: 0.51333033)

[casey-brooks]

Test & Lint Summary

• .venv/bin/pytest lib/matplotlib/tests/test_mlab_window_correction.py → 3 passed, 0 failed, 0 skipped
• .venv/bin/flake8 lib/matplotlib/tests/test_mlab_window_correction.py lib/matplotlib/mlab.py → no issues

[noa-lucent]

Changes align mlab's spectrum normalization with SciPy and include solid regression coverage for negative-weight windows. Looks good.