[ENH] Add Pesaran CD panel dependence diagnostic

[hass-nation]

Summary

• add a pesaran_cd cached result property to panel model results
• implement a standard-normal test statistic container for asymptotic z tests
• add balanced, unbalanced, and inapplicable-path coverage for the new diagnostic

Motivation

Pesaran's CD test is a widely used panel diagnostic for residual cross-sectional dependence, especially when users want a quick dependence check after fitting pooled, fixed-effects, or random-effects models. linearmodels already exposes panel result diagnostics and residual accessors, so this test fits naturally in the existing results API.

Reference

• Pesaran, M. H. (2021). "General diagnostic tests for cross-sectional dependence in panels." Empirical Economics, 60(1), 13-50.

Validation

• python -m pytest linearmodels/tests/shared/test_utility.py linearmodels/tests/panel/test_results.py
• python -m pytest linearmodels/tests/panel/test_pooled_ols.py::test_results_access linearmodels/tests/panel/test_panel_ols.py::test_results_access linearmodels/tests/panel/test_between_ols.py::test_results_access linearmodels/tests/panel/test_random_effects.py::test_results_access linearmodels/tests/panel/test_firstdifference_ols.py::test_results_access linearmodels/tests/panel/test_fama_macbeth.py::test_fama_macbeth linearmodels/tests/panel/test_fama_macbeth.py::test_fama_macbeth_kernel_smoke
• python -m ruff check linearmodels/shared/hypotheses.py linearmodels/panel/results.py linearmodels/tests/shared/test_utility.py linearmodels/tests/panel/test_results.py

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.52%. Comparing base (b535f55) to head (135602f).

Files with missing lines	Patch %	Lines
linearmodels/shared/hypotheses.py	90.62%	1 Missing and 2 partials ⚠️
linearmodels/panel/results.py	90.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #703      +/-   ##
==========================================
- Coverage   99.54%   99.52%   -0.03%     
==========================================
  Files         101      101              
  Lines       17426    17524      +98     
  Branches     1430     1435       +5     
==========================================
+ Hits        17347    17440      +93     
- Misses         29       31       +2     
- Partials       50       53       +3     

Flag	Coverage Δ
adder	99.50% <95.00%> (-0.03%)	⬇️
subtractor	99.50% <95.00%> (-0.03%)	⬇️

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