Fix SeLU lambda precision (Follow-up to #1287)

[valerioedu]

• Replaced literal cast 1.050700987… → res_T with a static const ap_fixed<16,1> 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.

[vloncar]

Can you add a test case that demonstrates that this is indeed working as expected and improves upon the current implementation? Should be straightforward

[valerioedu]

Can you add a test case that demonstrates that this is indeed working as expected and improves upon the current implementation? Should be straightforward

where should I commit the new test case?

[vloncar]

Look under tests/pytest