M14.4: as_chunks for the pixel loops main gained since the no-panics branch

Same treatment as the branch's `as_chunks` commit (Rust 1.99's
`chunks_exact_to_as_chunks` lint) for the flattener's coverage pass, the
CMYK ink compositor, the Flattener Preview highlight and four test helpers.

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

crates/engine/src/tests_objexpand.rs

@@ -69,7 +69,13 @@ fn mean_diff(a: &vectorcraft_render::Rendered, b: &vectorcraft_render::Rendered)
 
 /// Pixels (of 200×200) whose colour differs by more than `tol` in any channel.
 fn differing(a: &vectorcraft_render::Rendered, b: &vectorcraft_render::Rendered, tol: u8) -> usize {
-    a.pixels.chunks_exact(4).zip(b.pixels.chunks_exact(4)).filter(|(p, q)| p.iter().zip(q.iter()).any(|(x, y)| x.abs_diff(*y) > tol)).count()
+    a.pixels
+        .as_chunks::<4>()
+        .0
+        .iter()
+        .zip(b.pixels.as_chunks::<4>().0)
+        .filter(|(p, q)| p.iter().zip(q.iter()).any(|(x, y)| x.abs_diff(*y) > tol))
+        .count()
 }
 
 fn white_black() -> Value {