gamut-jpeg: XYB colour encode path

[justin13888]

• The encoder is hard-wired to T.871 §7 full-range BT.601 YCbCr; ChromaSubsampling (4:4:4 / 4:2:2 / 4:2:0) is the only colour lever on JpegEncoder (verified at c2df11b).
• This is the ceiling that keeps gamut-jpeg below jpegli's rate-distortion line irrespective of quantization tables (gamut-jpeg: quantization tables are frozen; no custom-table API #332) or trellis (gamut-jpeg: rate-distortion optimized coefficient selection (trellis + adaptive Q) #333) — those close the gap to mozjpeg, not to jpegli.
• Deliver: an XYB sample path alongside YCbCr, following jpegli's approach — XYB-coded samples plus an embedded ICC profile so any conformant decoder reproduces sRGB — selected by an encoder-level colour mode. JpegEncoder::with_icc_profile already exists and can carry the profile emission.
• Comparability note: https://github.com/justin13888/image-evaluation scores jpegli-encode xyb as a curated variant, so this is directly measurable against a reference the moment it lands.
• Related: Implement space-efficient JPEG encoder #19, Investigate and implement quality optimizations for JPEG encoder #29, Reference other high-quality JPEG encoders and see if they have any tricks we have not explored yet #30.