AVIF decode via gamut-avif container + hardware AV1

[justin13888]

• Blocked upstream: gamut-avif: container read surface + AV1 codestream handoff for external decoders gamut#250 (container read + av1C/OBU handoff — small surface, analogous to gamut#238 for HEIF). v1 blocker for AVIF decode.
• When it lands (pin bump first): AVIF decode = gamut-avif container surface + rawshift-hwdec AV1; replace the image crate's avif-native decode (avif-decode-image) and drop the image dep entirely.
• Add avif_hw_decode_available(); registry entries avif/videotoolbox|vaapi|mediacodec.
• Validate alpha, 10/12-bit, grid, and Exif/XMP item fixtures on hardware-capable runners.
• Software fallback (Windows/musl/wasm) is post-v1 via gamut-avif: pure-Rust AVIF (AV1) still decoder gamut#259.
• Depends on: Migrate AVIF encode to gamut-avif; delete libaom and ravif #26, Create rawshift-hwdec (HEVC+AV1); HEIC on gamut-heic; drop libheif-rs #27.