test(trace): keep the image crate's JPEG encoder for the block-artifact fixture

a_jpegs_blocks_are_not_traced_as_steps measures how many traced edges land
on JPEG block boundaries, which depends on the encoder's artifacts. With the
craft-image-codecs encoder 18 of 40 segments sit on block edges; with the
original encoder and the new shared decoder the test passes unchanged, so
the trace itself is unaffected. image stays a dev-dependency only.

Cargo.lock

@@ -7034,6 +7034,7 @@ name = "vectorcraft-trace"
 version = "0.24.1"
 dependencies = [
  "craft-image-codecs",
+ "image",
  "kurbo 0.13.1",
  "serde",
  "serde_json",

crates/trace/Cargo.toml

@@ -17,3 +17,5 @@ thiserror = { workspace = true }
 
 [dev-dependencies]
 serde_json = { workspace = true }
+# Test fixtures only: the JPEG-block test needs the block artifacts of the encoder it was tuned on.
+image = { workspace = true }

crates/trace/src/tests.rs

@@ -2174,12 +2174,14 @@ fn small_accents_of_other_colors_leave_a_tinted_photo_continuous_tone() {
     }
 }
 
-/// `img` saved as a JPEG of `quality` and read back.
+/// `img` saved as a JPEG of `quality` and read back. The `image` crate's encoder stays the fixture:
+/// `a_jpegs_blocks_are_not_traced_as_steps` measures block artifacts, which differ by encoder.
 fn through_jpeg(img: &Raster, quality: u8) -> Raster {
     let rgb: Vec<u8> = img.rgba.chunks(4).flat_map(|p| [p[0], p[1], p[2]]).collect();
-    let encoded = craft_image_codecs::Image::from_u8(img.width, img.height, craft_image_codecs::ChannelLayout::Rgb, rgb).unwrap();
-    let opts = craft_image_codecs::EncodeOptions { jpeg_quality: quality, ..Default::default() };
-    let bytes = craft_image_codecs::encode(&encoded, craft_image_codecs::Format::Jpeg, &opts).unwrap();
+    let mut bytes = vec![];
+    image::codecs::jpeg::JpegEncoder::new_with_quality(&mut bytes, quality)
+        .encode(&rgb, img.width, img.height, image::ExtendedColorType::Rgb8)
+        .unwrap();
     Raster::decode(&bytes).unwrap()
 }