Migrate PNG encode to gamut-png

[justin13888]

• Replace zune-png encode (png-encode-zune) with gamut-png; decode stays on zune-png until the gamut decoder lands (separate blocked issue) — encode-only swap, no dual encode backends.
• Embed metadata via PNG chunks (eXIf/iCCP/iTXt) through gamut, replacing img-parts PNG chunk muxing.
• Bench output size/speed vs the zune baseline (ref Measure gamut-png encoder efficiency gamut#224).
• Depends on: rawshift-core: replace generic primitives with gamut re-exports, keep sensor types #18, Migrate metadata stack to gamut-metadata #19.