perf(rust): reduce peak allocations when resizing cover artwork

Write JPEG resize output directly into RGB and release the original decoded image before encoding. Preserve PNG alpha handling and existing image quality.

Verified byte-identical output for 336 image fixtures and lower isolated peak heap allocation.

rust_backend/crates/core/src/cover.rs

@@ -73,7 +73,10 @@ pub fn resize<'a>(
         )
     };
     let source = Pixels::new(source);
-    let mut pixels = vec![0; dw as usize * dh as usize * 4];
+    // JPEG only consumes RGB. Write those channels directly instead of keeping
+    // an RGBA destination alongside a second RGB copy during encoding.
+    let channels = if png { 4 } else { 3 };
+    let mut pixels = vec![0; dw as usize * dh as usize * channels];
     // Match x/image/draw ApproxBiLinear: sample four neighbors at pixel centers
     // in premultiplied 16-bit space, then truncate into a premultiplied RGBA8
     // destination. See NOTICE.
@@ -90,8 +93,8 @@ pub fn resize<'a>(
                 source.at(x0, y1),
                 source.at(x1, y1),
             ];
-            let offset = (y as usize * dw as usize + x as usize) * 4;
-            for channel in 0..4 {
+            let offset = (y as usize * dw as usize + x as usize) * channels;
+            for channel in 0..channels {
                 let top = (1.0 - fx) * f64::from(samples[0][channel])
                     + fx * f64::from(samples[1][channel]);
                 let bottom = (1.0 - fx) * f64::from(samples[2][channel])
@@ -100,6 +103,8 @@ pub fn resize<'a>(
             }
         }
     }
+    // The original decode can be tens of MiB; encoding no longer needs it.
+    drop(source);
     check()?;
     let mut encoded = Vec::new();
     if png {
@@ -118,14 +123,8 @@ pub fn resize<'a>(
             .write_to(&mut Cursor::new(&mut encoded), ImageFormat::Png)
             .map_err(|error| format!("encode resized PNG artwork: {error}"))?;
     } else {
-        let rgb: Vec<_> = pixels
-            .as_chunks::<4>()
-            .0
-            .iter()
-            .flat_map(|pixel| pixel[..3].iter().copied())
-            .collect();
         image::codecs::jpeg::JpegEncoder::new_with_quality(&mut encoded, 88)
-            .encode(&rgb, dw, dh, image::ExtendedColorType::Rgb8)
+            .encode(&pixels, dw, dh, image::ExtendedColorType::Rgb8)
             .map_err(|error| format!("encode resized JPEG artwork: {error}"))?;
     }
     check()?;