Fix #320: CMYK images stay CMYK from PDF import to PDF export

A PDF's DeviceCMYK image opened as RGB (the interpreter's pixels as a PNG),
so every export wrote it as DeviceRGB or ICC RGB, silently. Images now keep
their CMYK samples, as CMYK colours already did:

- Model: CMYK images are blobs holding CMYK samples, a four-component JPEG
  (Adobe's inverted samples) or a Deflate CMYK TIFF. vectorcraft-doc's cmyk
  module reads their ink amounts (ImageBlob::is_cmyk, ::cmyk, capped at 64
  megapixels) and writes the TIFF (ImageBlob::cmyk_tiff).
- PDF import: DeviceCMYK, CalCMYK and four-component ICC images keep their
  samples. A JPEG with Decode [1 0 1 0 1 0 1 0] is kept byte for byte; any
  other is decoded (8 or 16 bits, its Decode array applied) and stored as a
  CMYK TIFF. Masked CMYK images and 1/2/4-bit ones open in RGB with a warning.
  Placing a CMYK TIFF keeps it CMYK too.
- PDF export: CMYK images are written in CMYK (DeviceCMYK, or ICC-based with
  the CMYK profile when colours are tagged: PDF/X-3, PDF/X-4, PDF/A). A CMYK
  JPEG neither resampled nor recompressed passes through unchanged; others are
  resampled in CMYK and compressed as ZIP or as a CMYK JPEG (jpeg-encoder,
  which also replaces the ZIP fallback and its warning for images converted to
  CMYK). Output conversion treats them like CMYK colours: Preserve Numbers and
  PDF/X-1a keep their numbers, Convert to Destination converts them to another
  CMYK profile or to RGB through a 17^4 lookup table (cms::CmykLut, now also
  behind the renderer's ink table). A CMYK image that can't be decoded is
  reported in the warnings.
- Canvas: CMYK images show through the working CMYK profile, as CMYK fills
  do, and paint their own inks in ink planes (CMYK raster exports, CMYK
  blending). SVG export writes TIFF images as PNG.

Illustrator's Output › No Conversion keeps colour data as it is
(https://helpx.adobe.com/illustrator/using/pdf-options.html).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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

@@ -282,13 +282,15 @@ pub(super) fn new_from_template(s: &mut Session, p: &Value) -> Result<Value> {
     open_template(s, src.name, &src.bytes, src.path)
 }
 
-/// Decode an image's header (and, for formats stored as PNG, its pixels).
+/// Decode an image's header (and, for formats stored as PNG, its pixels). CMYK TIFFs are kept as
+/// they are, with their ink amounts ([`ImageBlob::cmyk`]).
 pub fn raster_image(bytes: &[u8]) -> Result<RasterImage> {
     let reader = image::ImageReader::new(Cursor::new(bytes)).with_guessed_format().map_err(err)?;
     let kind = reader.format().ok_or_else(|| err("not an image VectorCraft reads (see document.formats)"))?;
     let f = image_format(kind).ok_or_else(|| err(format!("{kind:?} images can't be opened (see document.formats)")))?;
     let ppi = super::ppi::resolution(bytes);
-    let (bytes, mime, (width, height)) = if matches!(f.id, "png" | "jpg" | "gif" | "webp") {
+    let cmyk = || ImageBlob::new(f.mime, bytes.to_vec()).cmyk().is_some();
+    let (bytes, mime, (width, height)) = if matches!(f.id, "png" | "jpg" | "gif" | "webp") || (f.id == "tiff" && cmyk()) {
         (bytes.to_vec(), f.mime, reader.into_dimensions().map_err(err)?)
     } else {
         let img = reader.decode().map_err(err)?.to_rgba8();

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

@@ -103,6 +103,17 @@ fn webp_opens_at_its_pixel_size() {
     assert_eq!((r.width, r.height, r.blob.mime.as_str()), (3, 2, "image/webp"));
 }
 
+#[test]
+fn cmyk_tiffs_keep_their_inks_and_other_tiffs_become_png() {
+    let inks = vectorcraft_doc::cmyk::Inks::new(2, 1, vec![0, 102, 255, 0, 10, 20, 30, 40]).unwrap();
+    let tiff = vectorcraft_doc::ImageBlob::cmyk_tiff(&inks).unwrap();
+    let r = raster_image(&tiff.bytes).unwrap();
+    assert_eq!((r.width, r.height, r.blob.mime.as_str()), (2, 1, "image/tiff"));
+    assert_eq!(r.blob.cmyk(), Some(inks));
+    let rgb = raster_image(&image_bytes(2, 1, image::ImageFormat::Tiff)).unwrap();
+    assert_eq!(rgb.blob.mime, "image/png");
+}
+
 #[test]
 fn content_beats_a_wrong_extension() {
     let png = image_bytes(2, 2, image::ImageFormat::Png);