fix: return Ok after extracting kernel from APK

extract_kernel_from_apk fell through to an unconditional error after a
successful extraction, so every first --kernel run failed after
downloading the 45MB package; the second run worked from cache. Also
remove the downloaded APK even when extraction fails.

src/kernel.rs

@@ -224,12 +224,11 @@ fn download_alpine_kernel(branch: &str, arch: &str, dest: &Path) -> Result<()> {
     // Extract vmlinuz-virt from the APK
     // APK files are gzip-compressed tar archives
     let temp_apk = dest.with_extension("apk");
-    extract_kernel_from_apk(&temp_apk, dest)?;
-
-    // Clean up the APK
+    // Remove the ~45MB APK whatever the extraction outcome — don't leave
+    // it behind in the cache directory on failure.
+    let result = extract_kernel_from_apk(&temp_apk, dest);
     std::fs::remove_file(&temp_apk).ok();
-
-    Ok(())
+    result
 }
 
 async fn download_kernel_async(url: &str, dest: &Path) -> Result<()> {
@@ -316,6 +315,7 @@ fn extract_kernel_from_apk(apk_path: &Path, dest: &Path) -> Result<()> {
             // Extract to destination
             entry.unpack(dest).context("Failed to extract kernel")?;
             veprintln!("  Extracted kernel to: {}", dest.display());
+            return Ok(());
         }
     }