Module gui::gui_png

GUI: PNG image loading.

A real decoder — a from-scratch DEFLATE (RFC 1951) inflate implementation (stored/fixed-Huffman/dynamic-Huffman blocks, canonical Huffman decode, LZ77 back-references) plus PNG chunk parsing (RFC 2083) and the standard five scanline filters (None/Sub/Up/Average/Paeth) — not a stub. Scope: 8-bit-per-channel, non-interlaced PNGs with color type 0 (grayscale), 2 (RGB), or 6 (RGBA) — the overwhelming majority of PNGs in practice. Palette (color type 3), 16-bit depth, interlacing (Adam7), and ancillary chunks (gAMA, tRNS, etc.) are not implemented; GuiImage::load_png() fails cleanly (returns 0) rather than misdecoding them. Chunk CRCs are not verified (decoding, not validating, is the goal here).