gamut-png: implement spec-compliant PNG decoder

[justin13888]

• gamut-png currently only encodes; downstream rawshift is migrating fully onto gamut and must drop zune-png for decode.
• Deliver DecodeImage impls covering: 8/16-bit gray, gray-alpha, RGB, RGBA, and indexed (1/2/4/8-bit palettes), Adam7 interlacing, all standard filter types.
• Expose ancillary-chunk payloads for metadata as raw blocks consumable by gamut-metadata::MetadataBlock: eXIf, iCCP, iTXt (XMP), tEXt/zTXt, gAMA/cHRM/sRGB/cICP.
• Provide decode limits (max dimensions) for hostile-input safety.
• APNG out of scope.
• Related: Measure gamut-png encoder efficiency #224 (encoder efficiency), Stable Roadmap #168 (V1 roadmap). Blocks the rawshift PNG-decode migration.