gamut-ifd: RAW-grade parsing for camera TIFF files

[justin13888]

• Downstream rawshift is replacing its binrw-based TIFF parser with gamut-ifd as the engine under its ARW/CR2/CR3/NEF/DNG decoders.
• Parse from Read + Seek (or offset-preserving slices) of multi-hundred-MB camera files without loading everything.
• Follow SubIFD chains: SubIFDs (0x014A), Exif IFD, GPS IFD, maker-note offsets.
• Deterministic offset layout on the write path (DNG/TIFF export ordering).
• Hardening verification (bounds/cycle checks, byte completeness) is tracked separately as chore issues — this issue is the capability surface.
• Related: Stable Roadmap #168, Release gamut-tiff v1 #187. Blocks the rawshift TIFF-parser migration.