gamut-color: add BitDepth::Sixteen

[justin13888]

• BitDepth is Eight | Ten | Twelve (verified at 295c8924, crates/gamut-color/src/format.rs).
• Downstream rawshift standardizes on a 16-bit interleaved pipeline (PNG/TIFF/JXL/DNG 16-bit IO) and is replacing its own BitDepth (8/10/12/16) with the gamut-color type — blocked on a Sixteen variant.
• The enum is #[non_exhaustive]-friendly growth; bits()/max_value() extend naturally.
• Related: Stable Roadmap #168. Blocks the rawshift core-primitives migration slice for bit depth.

[justin13888]

Also consider other relevant bit depths that should exist (e.g. various RAW image formats using odd bit depths)