Let \( f : \mathbb{R} \to \mathbb{R} \) be a function defined as
\[
f(x) =
\begin{cases}
\dfrac{\sin\!\big((a+1)x\big) + \sin 2x}{2x}, & \text{if } x < 0, \\[6pt]
b, & \text{if } x = 0, \\[6pt]
\dfrac{\sqrt{x + b x^3} - \sqrt{x}}{b\, x^{5/2}}, & \text{if } x > 0.
\end{cases}
\]
If \( f \) is continuous at \( x = 0 \), then the value of \( a + b \) is equal to :