Print the black-and-white negatives instead of showing the scan

Choosing Ilford HP5 Plus showed an inverted grey frame. So did Double-X.
They are negatives, and upstream leaves `target_print` null on every
monochrome stock, so nothing was ever printed and the scan was all there was.

A colour negative at least announces itself -- the orange mask says plainly
that you are looking at a negative. A monochrome one just looks broken.

They print on Kodak 2302 now, which is a monochrome print film and is what
such a negative is actually printed onto; Double-X onto 2302 is the standard
cine chain. For the Ilford stocks it stands in for an Ilford paper, which
nobody has measured, and is at least the right kind of material.

The scan is still reachable through the Scanned/Printed toggle. It is a thing
to choose now rather than the only thing on offer.

`every_shipped_stock_bakes` did not catch this, and could not: it derives
"should this be inverted?" from the stock's kind *and whether it names a
paper*, so it looked at an inverted HP5, concluded that was right for an
unprinted negative, and passed. The assertion was self-consistent and the
situation was still wrong. The new test asserts the thing that actually
matters -- a camera negative must name a paper, that paper must be a printing
stock, and it must be the same kind of material, so a monochrome negative
cannot end up on colour paper.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/dr-film/src/lib.rs

@@ -153,6 +153,44 @@ mod tests {
         }
     }
 
+    #[test]
+    fn every_camera_negative_names_a_paper() {
+        // TRACES: FR-DEV-3f
+        // **The bug this exists to stop.** A negative with no paper renders as
+        // a negative: inverted, and for a black-and-white stock without even a
+        // colour negative's orange cast to say so. Choosing "Ilford HP5 Plus"
+        // and being shown an inverted grey frame reads as broken, not as a
+        // scan, and every black-and-white stock shipped that way because
+        // upstream leaves `target_print` null on all of them.
+        //
+        // The scan is still reachable — the Scanned/Printed toggle asks for it
+        // — but it is a thing to choose rather than the only thing on offer.
+        for film in camera_stocks() {
+            if film.kind != Kind::Negative {
+                continue;
+            }
+            let paper = default_print(film);
+            assert!(
+                paper.is_some(),
+                "{} is a negative and names no paper, so it renders inverted",
+                film.stock
+            );
+            let paper = paper.unwrap();
+            assert_eq!(
+                paper.stage,
+                Stage::Printing,
+                "{} names {} as its paper, which is not a printing stock",
+                film.stock,
+                paper.stock
+            );
+            assert_eq!(
+                paper.monochrome, film.monochrome,
+                "{} is printed on {}, which is the wrong kind of material",
+                film.stock, paper.stock
+            );
+        }
+    }
+
     #[test]
     fn a_projection_print_stock_is_not_offered_as_a_camera_film() {
         // Kodak 2383 is `support: film` and is nevertheless the Vision3