Date a DNG whose IFDs follow its pixels: read the head and the tail

The scan reads the first 256 KB of a file for its metadata. A camera
writes its IFDs at the front, so that is the whole structure; the linear
DNG a merge writes puts its first IFD after the pixels, and rawler,
given the head alone, finds no decoder in it. The composite was
catalogued without a date and sorted to the very end of the grid, after
every dated photograph — which is where a panorama merged on the tablet
went unfound.

dr-decode's own TIFF reader now reads through a head and a tail at a
known offset; trailing_ifd says where the tail starts and
metadata_split reads the two together. The scan, when the head fails
and points beyond itself, fetches from the IFD to the end — kilobytes —
and dates the file from both. Tested against the writer's own output.

core/dr-export/src/dng.rs

@@ -241,6 +241,8 @@ mod tests {
         let source = SourceMetadata {
             make: Some("Canon".into()),
             model: Some("Canon EOS 6D".into()),
+            captured_at: Some(1_754_398_664),
+            captured_offset: Some(120),
             ..Default::default()
         };
         write_linear_dng(
@@ -301,6 +303,32 @@ mod tests {
         assert_eq!((crop.p.x, crop.p.y, crop.d.w, crop.d.h), (2, 1, 15, 10));
     }
 
+    #[test]
+    fn the_catalog_reads_the_date_from_a_head_and_a_tail() {
+        // TRACES: FR-CAT-5
+        // The IFDs follow the pixels, so a scan that has the first bytes of
+        // the file has a pointer into nothing; rawler finds no decoder in
+        // that, and the composite would sit undated at the end of the grid.
+        // The scan's second range — from the first IFD to the end — with
+        // the head is enough to date it, and to name the camera.
+        let bytes = write(640, 400, 64);
+        let head = &bytes[..4096];
+        assert!(
+            dr_decode::metadata(head).is_err(),
+            "the head alone must not read"
+        );
+        let at = dr_decode::trailing_ifd(head).expect("the IFD is beyond the head");
+        assert!(at as usize > head.len());
+        let tail = &bytes[at as usize..];
+        assert!(tail.len() < 4096, "the tail is the IFD, not the pixels");
+        let md = dr_decode::metadata_split(head, tail, at).expect("read from two ranges");
+        assert_eq!(md.captured_at, Some(1_754_398_664));
+        assert_eq!(md.captured_offset, Some(120));
+        assert_eq!(md.model.as_deref(), Some("Canon EOS 6D"));
+        // A head that holds everything is not a trailing-IFD file.
+        assert_eq!(dr_decode::trailing_ifd(&bytes), None);
+    }
+
     #[test]
     fn a_strip_of_the_wrong_length_is_refused() {
         let mut bytes = std::io::Cursor::new(Vec::new());