A CMYK JPEG is not labelled JFIF

A CMYK JPEG came out with JFIF's APP0 segment at its head and four components
in the frame:

  APP0/JFIF  APP14/Adobe  APP2[ICC_PROFILE 1/4 … 4/4]  SOF0: 4 components

JFIF describes one- and three-component images, greyscale and YCbCr, so an APP0
segment in front of a four-component frame says two different things about the
file's colours: a reader that trusts JFIF decodes nonsense, and one that checks
refuses the file. Adobe's own CMYK JPEGs carry the APP14 marker the encoder
already writes, and no JFIF. `jpeg-encoder` writes JFIF unconditionally, so the
segment is replaced here.

The resolution JFIF carried moves to an EXIF APP1 segment -- `XResolution`,
`YResolution` and `ResolutionUnit` -- which is where `cmd::fileio::ppi` already
looks when a file has no JFIF, so nothing is lost; with no resolution asked for
the segment is simply dropped. RGB and greyscale files keep JFIF, which is what
it is for.

Found while looking at #3084, whose default RGB exports are valid here; this is
the file that is not.

crates/engine/src/cmd/fileio/ppi.rs

@@ -220,6 +220,21 @@ mod tests {
         assert!(close(resolution(&jpg).unwrap(), (254.0, 240.0 * 2.54)));
     }
 
+    /// A CMYK JPEG carries no JFIF (it describes one- and three-component images), so its
+    /// resolution is written as EXIF instead and this reader still finds it (#3084).
+    #[test]
+    fn a_cmyk_jpeg_keeps_its_resolution_without_jfif() {
+        use vectorcraft_render::encode::jpeg::{self, ColorModel, JpegOptions};
+        let o = JpegOptions { color_model: ColorModel::Cmyk, ..Default::default() };
+        let jpg = jpeg::encode(&[0; 4 * 4], 2, 2, 90, Some(240.0), &o).unwrap();
+        assert!(!jpg.windows(5).any(|w| w == b"JFIF\0"), "no JFIF on a CMYK file");
+        assert_eq!(resolution(&jpg), Some((240.0, 240.0)));
+        // An RGB file still says it in JFIF, as it always did.
+        let rgb = jpeg::encode(&[0; 3 * 4], 2, 2, 90, Some(240.0), &JpegOptions::default()).unwrap();
+        assert!(rgb.windows(5).any(|w| w == b"JFIF\0"));
+        assert_eq!(resolution(&rgb), Some((240.0, 240.0)));
+    }
+
     /// A little-endian TIFF/EXIF block declaring `x`/`y` resolution in `unit`.
     fn exif_block(x: u32, y: u32, unit: u16) -> Vec<u8> {
         let mut b = b"II*\0".to_vec();