Migrate DNG decode/encode to gamut-dng

[justin13888]

• Replace formats/dng.rs + formats/dng_export.rs with gamut-dng (DngDecoder/DecodedDng/DngEncoder); bridge gamut_dng::RawImage/CfaLayout/CameraProfile to rawshift_core::RawImage/ImageMetadata.
• Bridge-surface gaps tracked upstream in gamut-dng: public bridge surface for external RAW pipelines gamut#253 (per-cell black levels, LinearizationTable access, OpcodeList1/2/3 blobs, DNG 1.7 JXL tiles, public lossless-JPEG) — the affected slices wait on it.
• Replace codecs/ljpeg.rs on the DNG path with gamut-dng lossless-JPEG once public; keep local copy only while CR2/NEF still need it, then delete.
• DngExportConfig → DngEncodeConfig on the gamut-dng encoder.
• Validate against existing DNG fixtures; feed results to Validate DNG against real images gamut#174 (ref #109/#163).
• Depends on: TIFF-parser migration.