pyfixest-cupy64-absorbed-regressors

# pyfixest cupy64 backend: absorbed regressors survive, reports change shape ## Problem Adding `demeaner_backend="cupy64"` to an existing `pf.feols()` call is treated as a pure performance switch, but it changes the output: regressors with zero within-FE identifying variation (fully absorbed by the fixed effects) that the default numba backend silently drops are RETAINED by the cupy64 path (also in its scipy/CPU fallback when cupy is not installed). They appear in the coefficient table as non-identified garbage (huge coefficient, huge SE). ## Context / Trigger Conditions Observed 2026-07-18 (Specialist Directors US, H5 re-baseline audit): adding the kwarg to 4 feols sites left the headline triple stable to 4 decimals (B1 diff 1.45e-6, SE diff 9.9e-6) but the text report grew from 366 to 390 lines — 24 new rows for absorbed controls (e.g. `event_x_lrisk = 435.8059`, SE `7106.3365`) under firm-year + director FE, where firm-year-level variables have no identifying variation. ## Solution 1. Treat a backend change as a POTENTIALLY OUTPUT-CHANGING edit: diff the report and expect schema changes, not byte equality. Verify the coefficients of interest at ~4-decimal precision instead. 2. Never interpret retained absorbed-regressor rows as estimates; check within-FE variation before reading nuisance coefficients. 3. Never parse pyfixest text reports by line position; anchor on the variable name of the coefficient you need. 4. When byte-stable reports matter (regression-...