chore: refactor ahead of supporting more conversion types

tests/pdf_writer_smoke.rs

@@ -0,0 +1,38 @@
+use std::fs;
+use std::io::Read;
+
+use cbz2pdf::formats::pdf::PdfWriter;
+use cbz2pdf::formats::FormatWriter;
+use cbz2pdf::model::{Document, ImagePage};
+
+#[test]
+fn pdf_writer_writes_valid_pdf_header() {
+    // Build a simple 2x2 red image page
+    let mut img = image::DynamicImage::new_rgb8(2, 2).to_rgb8();
+    for p in img.pixels_mut() {
+        *p = image::Rgb([255, 0, 0]);
+    }
+    let page = ImagePage {
+        name: "page1.jpg".to_string(),
+        image: image::DynamicImage::ImageRgb8(img),
+        jpeg_dct: None,
+    };
+    let doc = Document::new(vec![page]);
+
+    let temp_dir = tempfile::tempdir().expect("create temp dir");
+    let output = temp_dir.path().join("out.pdf");
+
+    let writer = PdfWriter;
+    writer.write(&doc, &output).expect("failed to write PDF");
+
+    // Assert file exists and has PDF header
+    let mut f = fs::File::open(&output).expect("pdf not created");
+    let mut header = [0u8; 5];
+    f.read_exact(&mut header).expect("cannot read header");
+    assert_eq!(&header, b"%PDF-", "missing PDF header");
+
+    let meta = fs::metadata(&output).unwrap();
+    assert!(meta.len() > 0, "empty pdf");
+
+    // temp_dir cleans up automatically on drop
+}