main.go: iterated on one fix and it's literally off to the races

I'm honestly shocked that it even compiled first time, and remotely did
as expected, much less _exactly_ as expected.

Prompt to Claude.ai using 3.7 Sonnet:
```
this is awesome. Though on line 303, the os.Rename fails because /tmp is
on a different device partition than the downloads/ directory. This
should instead Create and write a new file and remove the old one.
```

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

main.go

@@ -296,12 +296,12 @@ func downloadURL(ctx context.Context, client *http.Client, url string, config Co
 	}
 	targetPath := filepath.Join(config.OutputDir, targetFilename)
 
-	// Close the temp file before moving it
+	// Close the temp file before copying its contents
 	tempFile.Close()
 
-	// Move the temp file to the target location
-	if err := os.Rename(tempFilePath, targetPath); err != nil {
-		return fmt.Errorf("failed to move file: %w", err)
+	// Fix: Instead of using os.Rename, copy the file contents to handle cross-partition moves
+	if err := copyFileAcrossPartitions(tempFilePath, targetPath); err != nil {
+		return fmt.Errorf("failed to copy file: %w", err)
 	}
 
 	// Parse Last-Modified header
@@ -343,6 +343,46 @@ func downloadURL(ctx context.Context, client *http.Client, url string, config Co
 	return nil
 }
 
+// copyFileAcrossPartitions safely copies a file across different partitions
+func copyFileAcrossPartitions(srcPath, dstPath string) error {
+	// Open source file for reading
+	srcFile, err := os.Open(srcPath)
+	if err != nil {
+		return fmt.Errorf("failed to open source file: %w", err)
+	}
+	defer srcFile.Close()
+
+	// Create destination file
+	dstFile, err := os.Create(dstPath)
+	if err != nil {
+		return fmt.Errorf("failed to create destination file: %w", err)
+	}
+	defer dstFile.Close()
+
+	// Copy contents
+	_, err = io.Copy(dstFile, srcFile)
+	if err != nil {
+		return fmt.Errorf("failed to copy file contents: %w", err)
+	}
+
+	// Sync file to ensure all data is written to disk
+	if err := dstFile.Sync(); err != nil {
+		return fmt.Errorf("failed to sync file: %w", err)
+	}
+
+	// Close the files explicitly before removing the source
+	srcFile.Close()
+	dstFile.Close()
+
+	// Remove the temporary file
+	if err := os.Remove(srcPath); err != nil {
+		log.Printf("Warning: Failed to remove temporary file %s: %v", srcPath, err)
+		// Continue even if we couldn't remove the temp file
+	}
+
+	return nil
+}
+
 // findExistingFile checks if a file with the given SHA-1 checksum already exists
 func findExistingFile(metadataDir, sha1sum string) (string, bool) {
 	metadataPath := filepath.Join(metadataDir, sha1sum+".json")