Make the view transform an operation the photographer can set

FR-DEV-3j gives the view transform two controls, contrast and the white
point in stops above middle grey, persisted and held per mask layer like
any other setting. A scene-referred pipeline whose white point cannot be
moved hands the photographer a shoulder they cannot place.

`view_transform` is a hand-written node in `Stage::View`, a new stage
the composer emits last and emits whatever the node's state: a neutral
operation is otherwise left out of the shader, but a photograph with no
view transform is a scan. "Active" keeps meaning "moved from the
defaults", so an untouched photograph writes nothing for it and
`every_node_starts_neutral` still holds. A caller whose chain holds no
view operation gets a default one.

The composer's loop becomes an ordered list of steps — camera nodes, the
matrix, scene nodes, layer-only nodes, the view — so each is emitted in
exactly one place. The base curve could not be a node because it
belonged to the camera; the ops README records why that argument went
with it (D19). The panel shows it in the Light group as "Tone Mapping".

Tests that counted the blocks of a neutral graph now count one, the
view transform, and the two chain-wide tests that load a film expect the
view transform to be absent, since a stock replaces it.

core/dr-pipeline/src/graph.rs

@@ -1122,13 +1122,21 @@ mod tests {
     fn a_fresh_graph_is_neutral() {
         // Opening an unedited image must produce the image, not an
         // interpretation of it.
+        //
+        // One block, and it is the view transform: a view operation is
+        // composed at its defaults, because a photograph with no view
+        // transform is a scan rather than a picture (FR-DEV-3j). It is still
+        // neutral in the sense that matters here — nothing moved, nothing is
+        // written — and every adjustment is absent.
         let g = EditGraph::default_chain();
         assert!(g.is_neutral());
+        let source = g.compose().source;
         assert_eq!(
-            g.compose().source.matches("---- ").count(),
-            0,
-            "a neutral graph must generate no operation blocks"
+            source.matches("---- ").count(),
+            1,
+            "a neutral graph must generate no adjustment blocks"
         );
+        assert!(source.contains("---- view_transform ----"));
     }
 
     #[test]
@@ -1207,13 +1215,15 @@ mod tests {
     #[test]
     fn only_active_operations_reach_the_shader() {
         // The composition property, end to end: two adjustments out of seven
-        // available must generate a shader doing exactly two things.
+        // available must generate a shader doing exactly two things — and
+        // the view transform, which every render has (FR-DEV-3j).
         let mut g = EditGraph::default_chain();
         g.set_param(exposure::ID, exposure::EXPOSURE, 1.0);
         g.set_param(white_balance::ID, white_balance::TINT, 25.0);
 
         let shader = g.compose();
-        assert_eq!(shader.source.matches("---- ").count(), 2);
+        assert_eq!(shader.source.matches("---- ").count(), 3);
+        assert!(shader.source.contains("---- view_transform ----"));
         assert!(shader.source.contains("---- exposure ----"));
         assert!(shader.source.contains("---- white_balance ----"));
         assert!(!shader.source.contains("---- saturation ----"));