Quantized row-sum shell index

Fix a shell half-width $\Delta>0$. For a row $a$ and coloring $\omega$, $\mathrm{shellIdx}(\Delta,a,\omega)$ is the integer $j$ nearest to $\mathrm{rowSumB}(a,\omega)/(2\Delta)$, i.e. it identifies which width-$2\Delta$ "shell" $[\,(2j-1)\Delta,(2j+1)\Delta)$ the row sum falls into. This quantization is the key device in the entropy-method proof of Spencer's theorem (Rothvo\u00df, Lemma 9): two colorings landing in the same shell have row sums within $2\Delta$ of each other.