Fix fixest iv path

[DGoettlich]

Summary: fixes IV support for fixest.

Issue

• scpcR was checking model$iv to decide whether a fixest model was an IV second-stage fit.
• Somtime between v0.12 and v0.14 fixest, that flag seems to have been changed to model$is_iv instead.
• Because of that, IV models were not recognized correctly, and scpcR fell back to the ordinary model-matrix path. That then caused a name mismatch like x vs fit_x and made the IV branch fail.

Fix

• The fix updates the IV detector to use model$is_iv, keeps the stage check via iv_stage, and pins fixest >= 0.14.0 in the package metadata.
• dded a regression test for IV-stage detection.
• the existing IV helper tests, IV smoke tests, and the full test suite pass again.

Testing

• Rscript -e "devtools::test(stop_on_failure = FALSE)"