ppm_writer.nx

ppm_writer.nx -- serialise Netpbm images (PBM / PGM / PPM). The Netpbm family is the simplest lossless image format alive: ASCII magic + dimensions + max-value + raw binary pixels. Used as a debug output target by virtually every graphics engine during bring-up (ImageMagick, pixman, cairo, mesa) precisely because you can emit it with printf. Magic codes: P1 ASCII PBM 1-bit bitmap (0/1) P2 ASCII PGM grayscale P3 ASCII PPM RGB P4 binary PBM P5 binary PGM P6 binary PPM We emit P5 (binary grayscale) + P6 (binary RGB) -- the two forms useful for rendering output. ASCII forms are larger and slower. Binary PBM (P4) packs 8 pixels per byte and is rarely worth the complexity. Use cases: Nishi game engine screenshot dump, procgen debug visualisations, DDS/PNG precursor, comparing frame buffers byte-exact in regression tests. Invariants: P1 Header is plain ASCII, LF-terminated fields. P2 max_value literal: we hardcode 255 (8-bit per channel). P3 Binary pixel block: P5 width*height bytes, P6 width*height*3 bytes, row-major, top-to-bottom.