What is the article-lām rule in «الشِّتَاءِ»?

Al-Fīl and Quraysh: Applied Iẓhār, Ikhfāʾ, Idghām, Madd and Waqf

Nūn assimilation without ghunnah
Sun-letter lām, assimilating into shīn
Moon-letter lām, pronounced
Ikhfāʾ shafawī