AVIF/HEIC decode (opt-in compile flag, read-only)

[swperb]

Purpose: read modern iPhone/web delivery formats and convert them out.
Ease: 🔴 Hard — AVIF=AV1-intra, HEIC=HEVC-intra; both need heavy libs (libavif/libheif), and HEIC carries patent baggage. Not hand-rollable / not in stb.
Decision: opt-in only — the default build stays zero-dependency.

Tasks

• make AVIF=1 links libavif for decode; default build unaffected
• read-only convert path (imgcli -i in.avif out.png)
• HEIC behind its own flag (patents) — evaluate separately
• document the opt-in clearly so the zero-dep claim stays honest