M8: the JBIG2 redaction test compares the whole saved picture

One eight-byte search would miss a clearing that took only the first byte
of it: the window is gone and seven covered bytes stay. The saved picture
now has to be the one with every pixel under the mark cleared and no other
changed, on both coded forms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AmsdeUCdTaD72jkDw2pJKY

crates/redact/src/tests.rs

@@ -740,6 +740,8 @@ fn a_busy_fax_image_behind_another_filter_is_cleared() {
 /// on a scan coded with symbols and on one coded as a generic region. `COVERED` and `KEPT` are pixels
 /// found nowhere else in the image, one lot under the mark and one beside it, so the search is seen
 /// to find what stays. The one letter over and over, lower down, is what makes symbols worth coding.
+/// The picture saved is the one with every pixel under the mark cleared and no other changed, so a
+/// search that one cleared byte would blind isn't all that holds this.
 #[test]
 fn no_stream_holds_the_pixels_under_a_mark_on_a_jbig2_scan() {
     const COVERED: [u8; 8] = [0x0F, 0x3C, 0x5A, 0xC3, 0x96, 0x69, 0xA5, 0x1E];
@@ -765,6 +767,12 @@ fn no_stream_holds_the_pixels_under_a_mark_on_a_jbig2_scan() {
             _ => false,
         })
     };
+    // Columns 0 to 99 of rows 0 to 7.
+    let mut cleared = bits.clone();
+    for row in cleared.chunks_mut(stride).take(8) {
+        row[..12].fill(0);
+        row[12] &= 0x0F;
+    }
     let symbols = pdfcraft_filters::encode_jbig2(&bits, w as u32, h as u32).unwrap();
     let generic = pdfcraft_filters::encode_jbig2_generic(&bits, w as u32, h as u32).unwrap();
     assert!(symbols.len() < generic.len(), "symbol coding has to be what this tests");
@@ -773,12 +781,12 @@ fn no_stream_holds_the_pixels_under_a_mark_on_a_jbig2_scan() {
         // Half a point a pixel, its top row at y 132.
         let mut doc = one_page(b"q 256 0 0 32 0 100 cm /Im Do Q", "/XObject << /Im 7 0 R >>", vec![stream(&dict, coded)]);
         assert!(holds(&doc, &COVERED) && holds(&doc, &KEPT), "the pixels are there to be found");
-        // Columns 0 to 99 of rows 0 to 7.
         mark(&mut doc, 0, &[[0.25, 128.25, 49.75, 132.0]], "");
         assert_eq!(apply(&mut doc, None).unwrap().images_cleared, 1);
         let saved = Document::open(Arc::new(write_full(&doc, &SaveOptions::default()).unwrap())).unwrap();
         assert!(!holds(&saved, &COVERED), "the covered pixels stayed in the file");
         assert!(holds(&saved, &KEPT), "the pixels beside them stay");
+        assert!(holds(&saved, &cleared), "every pixel under the mark is cleared, and no other changed");
     }
 }