QOI read/write (zero-dependency)

[swperb]

Purpose: fun, fast, lossless, dead-simple modern format — a pure zero-dep win and a nice "imgcli speaks QOI" post.
Ease: 🟢 Easy — the QOI spec is ~one page; a reference codec is ~300 lines, public domain.

Tasks

• implement (or vendor) QOI encode + decode
• .qoi in img_load/img_save
• tests + README; add a QOI-vs-PNG size/speed row to bench/

Note: zero-dep — fits the wedge perfectly.

[swperb]

Implementation notes (cleanest possible fit)

• Spec: https://qoiformat.org (one page). The reference single-header qoi.h is public domain — vendor it in third_party/ exactly like stb (#define QOI_IMPLEMENTATION in one TU, i.e. image.c), or implement ~300 lines.
• QOI is 8-bit RGB/RGBA → maps 1:1 onto our packed RGBA8 Image. No pixel-format conversion needed.
• Wire .qoi into the img_load/img_save dispatch in src/image.c. Use qoi_read/qoi_write (or the *_from_memory/*_to_func variants for the in-memory path + fuzzing).
• Security: validate the QOI header width/height with img_dims_ok() before allocating (decompression-bomb guard), same as img_load. Add the .qoi path to the fuzz harness.
• Bonus post material: add a QOI vs PNG size + encode-speed row to bench/.