Same model as Fig. 1. The figure shows the core structure of the progenitor at the onset of core collapse. Panel (a): radial infall velocity (black dotted line, left axis) and mass fraction profiles of ⁵⁶Fe, ²⁸Si, and ¹⁶O (dashed lines, right axis) versus the mass coordinate M_r. Panel (b): radial profiles of the central density (dashed line) and specific entropy (dotted line) as functions of the mass coordinate M_r, where the iron core mass is 1.85 M_⊙. The dash-dotted blue line marks the mass coordinate where the central specific entropy s_c = 4 k_B, corresponding to the Si/O shell interface at M_r = 2.32 M_⊙.