Step 1: Observe the Symmetry
The two fractions $\dfrac{x^{2}}{x^{2}+(2026-x)^{2}}$ and $\dfrac{(2026-x)^{2}}{x^{2}+(2026-x)^{2}}$ add up to $1$.
Step 2: Use the Limits
The interval is symmetric about $x=1013$, so both integrals are equal and each is half the integral of $1$, i.e. $\dfrac{2026-2\alpha}{2}$.
Step 3: Solve
$1013-\alpha=100$, so $\alpha=913$.
Final Answer:
Solving 1013 - alpha = 100 gives alpha = 913.
\[ \boxed{\text{(A) }913} \]