Migrate PNG decode to gamut-png

[justin13888]

• Blocked upstream: gamut-png: implement spec-compliant PNG decoder gamut#249. v1 blocker — PNG is in default features.
• When it lands (pin bump first): replace zune-png decode (png-decode-zune).
• Drop zune-png; if this is the last zune codec (after JXL/JPEG migrations), drop zune-core + the zune-runtime feature — but zune-ppm stays (permanent exception); check whether it needs zune-core.
• Ancillary-chunk metadata (eXIf/iCCP/iTXt) flows through gamut-metadata.
• Depends on: Migrate PNG encode to gamut-png #23.