`signal`: `*_zpk` should propagate `float32` input, not promote to `float64`

[jorenham]

Claude Fable 5 reports:

• signal/_filter_design.pyi:422-560 bilinear_zpk/lp2hp_zpk/lp2lp_zpk f32 overloads claim f64 (or float k); runtime keeps float32 for p/k. Analog bessel/ellip/iirfilter zpk k is builtin float, claimed np.float64 (:649-882).