…arning#1287)

* Replaced literal cast 1.050700987… → `res_T` with a
  `static const ap_fixed<16,6> lambda`, preserving range and
  ~1.5 × 10⁻² LSB precision regardless of user-chosen `res_T`.
* Removed redundant datareg scope; now a single per-element branch:
      if (x ≥ 0)  y = λ · x
      else        y = selu_table[idx]   (with index clamped to [0,N-1]).
* Guard against negative-index underflow; behaviour for <0 inputs unchanged.
* Keeps identical latency/area on Vivado 2023.1 & Vitis 2024.1, but
  eliminates silent overflow/rounding when `res_T` is narrow (e.g., ap_fixed<8,2>).

Fixes fastmachinelearning#1287