Defines the shape of islands that appear in the river. Each island is 3 bytes. Referenced from terrain fragment byte 3 (upper 6 bits = island index × 4). Initialized by handle_island.

Byte	Contents
0	Profile index (0-14): selects shape from data_terrain_profiles
1	Width offset: added to profile values, shifts edge position
2	Edge mode: 0=use byte directly, 1=mirror, 2=offset from left

Edge mode controls right edge calculation in render_island_line: mode 1 creates symmetric islands (right = 120 - left), mode 2 creates fixed-width channels (right = 60 + left).