Give the preflight test fixtures a PDF/A-2 file header

The header check reports a file without the binary comment, and Save as PDF/A then makes the next save a full one. The known-answer documents and the generation-repair file predate it: the minimal document got a 6.1.2 issue, and an undamaged file needed a full save for its header instead of being saved incrementally. They now start with the comment, as the crate's unit test documents do.

crates/preflight/tests/generation_repairs.rs

@@ -4,7 +4,9 @@ use pdfcraft_preflight::{Level, convert};
 use std::sync::Arc;
 
 fn image_file(damaged: bool) -> Arc<Vec<u8>> {
-    let mut bytes = b"%PDF-1.7\n".to_vec();
+    // A PDF/A-2 header (6.1.2), so that Save as PDF/A has no header to rewrite and only the
+    // repaired references decide whether the next save is full.
+    let mut bytes = b"%PDF-1.7\n%\xE2\xE3\xCF\xD3\n".to_vec();
     let bodies = [
         "<< /Type /Catalog /Pages 2 0 R /HeaderImage 5 1 R /RowImage 5 0 R /StaleImage 5 2 R >>",
         "<< /Type /Pages /Kids [3 0 R] /Count 1 >>",

crates/preflight/tests/known_answers.rs

@@ -22,7 +22,8 @@ use pdfcraft_preflight::{Level, convert, declared, icc, verify};
 /// A document from hand-written objects and extra trailer entries (copied from the crate's unit
 /// tests).
 fn build(objs: &[&str], trailer_extra: &str) -> Document {
-    let mut out = b"%PDF-1.7\n".to_vec();
+    // ISO 19005-2 6.1.2: the header line is followed by a comment of four bytes above 127.
+    let mut out = b"%PDF-1.7\n%\xE2\xE3\xCF\xD3\n".to_vec();
     let mut offsets = Vec::new();
     for (i, o) in objs.iter().enumerate() {
         offsets.push(out.len());