[CodingSelim]

when the regressors include a dummy for every group, the IV model F test currently tests all coefficients against zero, including the intercept those dummies span. it should test that the group effects are equal and the other slopes are zero. this also makes the result depend on whether the same model is written with an explicit intercept. the example in #780 changes from 227.678 on 3 degrees of freedom to the intended 10.775 on 2.

this finds the coefficient direction that produces the implied intercept and tests the remaining directions, using the fitted covariance and the existing F/chi-square calculation. the columns are scaled before finding that direction so differently sized dummy columns give the same test.

related to #780. this handles the IV estimators; the panel and system cases in that issue still need their own changes.

added 48 regression cases covering IV2SLS, IVLIML and IVGMM, unadjusted/robust covariance, debiased on/off, weights, and differently scaled dummies. each checks against explicit Wald restrictions and the equivalent explicit-intercept model. the new test fails on main before the fix. also checked a mixed-column design with IV2SLS, IVGMMCUE and IVJIVE.

validation:

• IV tests plus the singular-F-statistic regression module: 9582 passed, 2400 skipped.
• ruff check on the changed files, formatting of the new test file, and git diff --check pass.
• built and tested the current checkout in an editable install. the whole-repository run was stopped after the IV run completed, so there is no full-suite result.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.68%. Comparing base (7fa390b) to head (28bed31).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #859   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files         199      200    +1     
  Lines       37875    37909   +34     
  Branches     2436     2437    +1     
=======================================
+ Hits        37756    37790   +34     
  Misses         51       51           
  Partials       68       68           

Flag	Coverage Δ
adder	99.67% <100.00%> (+<0.01%)	⬆️
subtractor	99.67% <100.00%> (+<0.01%)	⬆️

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