Quartz::PNG::DecodedImage

Minimal PNG decoder for the small icons FAP authors ship. Supports bit depth 8 with color types 0 (gray), 2 (RGB), 4 (gray+alpha), and 6 (RGBA). Palette (3) and 16-bit depth aren't used by Flipper icon tooling — we raise rather than silently misdecode.

This is just enough for Quartz::PNG.decode_icon to produce a 1bpp bitmap; it is not a general-purpose decoder.