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/cmd/flatten.rs

@@ -938,7 +938,7 @@ fn render_raster(doc: &Document, art: &[Node], rect: Rect, ppi: f64, clip: Optio
             })
             .collect();
         let cover = r.render_region(&isolated_doc(doc, cover), region, scale, false);
-        for (px, c) in img.pixels.chunks_exact_mut(4).zip(cover.pixels.chunks_exact(4)) {
+        for (px, c) in img.pixels.as_chunks_mut::<4>().0.iter_mut().zip(cover.pixels.as_chunks::<4>().0) {
             let k = c[3];
             let k2 = if o.anti_alias || k == 0 || k == 255 {
                 k