Drop a within-entity constant when an intercept is present

[shaneraphel]

Summary

With entity effects and an intercept, a regressor that does not vary within an entity is demeaned to zero and then restored as its grand mean. not_absorbed residualizes that column on the intercept. Exact arithmetic leaves nothing, but the eigenvalue tolerance is taken relative to the remaining roundoff, so the column is kept. On the bundled wage panel, education is constant for every worker. lwage ~ 1 + educ + EntityEffects with drop_absorbed=True reported 0.13914557037073996. Replacing education by education minus 12 reported -0.36450348040229325. Both p-values were 0. The same model with union included already dropped education and kept union at 0.07468459435240984. drop_absorbed=False already raised AbsorbingEffectError.

The extra check compares the residual norm with the column's own norm, and it returns only the intercept when every non-constant column vanishes. A regression that still has a within-entity regressor uses the existing rank test. After the change both encodings drop the time-invariant column and return the same intercept, 1.6491471921100918.

Fixes #728.

This patch was prepared with the help of an AI assistant. I compared the two encodings of education on the bundled wage panel, and the regression that also includes union.

Test plan

• pytest linearmodels/tests/panel/test_panel_ols.py::test_drop_absorbed_constant_when_regressor_is_time_invariant linearmodels/tests/panel/test_panel_ols.py::test_fully_absorbed linearmodels/tests/panel/test_panel_ols.py::test_absorbed (4 passed on this branch)
• Wage panel: both encodings keep only the intercept 1.6491471921100918
• union stays at 0.07468459435240984 and education is still dropped
• drop_absorbed=False still raises AbsorbingEffectError

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.53%. Comparing base (ec0f288) to head (c383283).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #730      +/-   ##
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- Coverage   99.54%   99.53%   -0.02%     
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  Files         101      101              
  Lines       17450    17476      +26     
  Branches     1433     1435       +2     
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+ Hits        17371    17394      +23     
- Misses         29       31       +2     
- Partials       50       51       +1     

Flag	Coverage Δ
adder	99.51% <100.00%> (-0.02%)	⬇️
subtractor	99.51% <100.00%> (-0.02%)	⬇️

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