FLINT polynomial-multiplication check, 2026-10-12 (python-flint 0.9.0, FLINT 3.6.0)

1. Signed-FFT regression test from OpenAI's prime_gap_186_certificate.py (check_flint_signed_fft):
   result: FAIL, wrong coefficients [15]
   (shim) _fmpz_poly_mul           calls=1 maxlen=16
   (shim) _fmpz_poly_mul_SS        calls=1 maxlen=16

2. Complete runs of certify/reproduce.py under certify/flint_polymul_shim.c:
   FLINT_LIB=<libflint of python-flint> LD_PRELOAD=./flint_polymul_shim.so python3 reproduce.py 2 3
   SHIM _fmpz_poly_mullow        calls=0 maxlen=0
   SHIM _fmpz_poly_mul           calls=0 maxlen=0
   SHIM _fmpz_poly_sqrlow        calls=0 maxlen=0
   SHIM _fmpz_poly_sqr           calls=0 maxlen=0
   SHIM _arb_poly_mullow         calls=0 maxlen=0
   SHIM _acb_poly_mullow         calls=0 maxlen=0
   SHIM _arb_poly_mullow_block   calls=0 maxlen=0
   SHIM _fmpz_poly_mullow_SS     calls=0 maxlen=0
   SHIM _fmpz_poly_mul_SS        calls=0 maxlen=0
   SHIM _fmpz_poly_mullow_KS     calls=0 maxlen=0
   SHIM _fmpz_poly_mul_KS        calls=0 maxlen=0
   SHIM _fmpz_poly_mulmid        calls=0 maxlen=0
   FLINT_LIB=<libflint of python-flint> LD_PRELOAD=./flint_polymul_shim.so python3 reproduce.py 4 5
   SHIM _fmpz_poly_mullow        calls=0 maxlen=0
   SHIM _fmpz_poly_mul           calls=0 maxlen=0
   SHIM _fmpz_poly_sqrlow        calls=0 maxlen=0
   SHIM _fmpz_poly_sqr           calls=0 maxlen=0
   SHIM _arb_poly_mullow         calls=0 maxlen=0
   SHIM _acb_poly_mullow         calls=0 maxlen=0
   SHIM _arb_poly_mullow_block   calls=0 maxlen=0
   SHIM _fmpz_poly_mullow_SS     calls=0 maxlen=0
   SHIM _fmpz_poly_mul_SS        calls=0 maxlen=0
   SHIM _fmpz_poly_mullow_KS     calls=0 maxlen=0
   SHIM _fmpz_poly_mul_KS        calls=0 maxlen=0
   SHIM _fmpz_poly_mulmid        calls=0 maxlen=0

   The certificate values printed in these runs are identical to logs/reproduce.log.