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/lib.rs

@@ -169,6 +169,13 @@ mod tests {
         let mut fused_blocks = 0;
         for desc in g.descriptors() {
             let id = desc.id.0;
+            // The film is loaded here, and a stock is a rendering: the view
+            // transform it replaces is correctly in neither stage (FR-DEV-3f,
+            // FR-DEV-3j).
+            if id == crate::ops::view_transform::ID.0 {
+                assert!(!shader.source.contains("---- view_transform ----"));
+                continue;
+            }
             let point = shader.source.contains(&format!("---- {id} ----"));
             let neighbourhood = detail
                 .passes