feat(png)!: migrate PNG decode to gamut-png; drop zune-png - #53
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PNG decode moves to gamut_png::PngDecoder's rich decode() path, normalised to interleaved Rgb16: 16-bit sources pass through natively and sample-exact (unit-pinned), 8-bit scales x257, grayscale replicates, palettes expand, and alpha drops — all verified at parity with the old zune behaviour. The typed DecodeImage impls were deliberately not used for the main path: they are lossless-only by contract and refuse RGBA/tRNS sources, while rawshift's contract is decode-anything-drop-alpha; decode() covers the full domain through pure gamut API. ZunePngDecodeConfig becomes PngDecodeConfig exposing exactly gamut's resource guards (max dimensions, max image bytes, max metadata bytes — DoS protection the JPEG decoder currently lacks upstream, ref gamut#306). The old confirm_crc/strict knobs are gone: gamut always verifies critical-chunk CRCs and skips corrupt ancillary chunks per spec. DecodeOptions::PngZune -> Png; codec id png/gamut on both directions now. PNG metadata reads now source EXIF, ICC, and XMP from DecodedPng's typed fields where the old path scanned eXIf alone — the hand-rolled chunk scanner and ExifContainer::Png are deleted. zune-core/zune-runtime stay only for the permanent zune-ppm exception; zune-png leaves the lockfile (zune-jpeg remains transitively via resvg/tiff until their migrations). BREAKING CHANGE: PNG decode config/variant renames; confirm_crc/strict decode knobs removed.
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Closes #32.
Stacked on #52; merge order #42 → … → #52 → this. PNG is now gamut end-to-end.
What is Delivered
decode()path, normalised toRgb16: 16-bit native passthrough (sample-exact, unit-pinned), 8-bit ×257, grayscale replication, palette expansion, alpha dropped — parity with zune verified case by case. The typedDecodeImageimpls were deliberately bypassed for the main path: they're lossless-only by contract (refuse RGBA/tRNS), while rawshift's contract is decode-anything-drop-alpha. Pure gamut API either way.PngDecodeConfigexposes exactly gamut's resource guards (max dimensions / image bytes / metadata bytes). The oldconfirm_crc/strictknobs are gone — gamut always verifies critical CRCs.DecodeOptions::Png; codec idpng/gamutboth directions.DecodedPng's typed fields (old path: eXIf only). Hand-rolled chunk scanner andExifContainer::Pngdeleted.zune-pngout of the lockfile;zune-core/zune-runtimeremain solely for the permanentzune-ppmexception; transitivezune-jpegheld only byresvg/tiffuntil their migrations.Parity note
Sub-byte grayscale (1/2/4-bit) now scales exactly per PNG §13.12; if zune differed, gamut is the spec-correct presentation.
Verification Done
Run independently after implementation:
cargo test --workspace387/0;--features full544/0 (all 5 PNG fixture goldens incl. pixel values);--features serde360/0; clippy workspace + full-D warnings0+0;--no-default-features --features pngstandalone clean;zune-pngabsent from the lockfile. A png-only test-target compile failure (ungated tiff/gif helpers) was confirmed pre-existing viagit stash. lefthook green.Upstream follow-up (not filed yet)
gamut-png lacks a metadata-only entry point (gamut-jpeg has
metadata()), so PNG metadata reads pay a guarded full decode — worth an upstream issue.